Step one:
Remove the slide from the frame.
Remove barrel and recoil spring.
Turn the slide over. Looking at it you'll see at the rear, the striker "dog leg". Look closely, you'll see the gap at the front and see a part of the striker spring, also there will be a bit of white plastic showing at the front of the gap.
Insert a small punch into the gap, at the edge of the white plastic and push forward to relieve pressure off the striker back plate on the rear of the slide. You will now be able to remove the back plate. The striker will then pop out a bit.
Look to the right side underneath of the slide and you'll see a small round silver disk. This is the striker safety.
Push this in to release the striker.
Remove the striker from the slide. The striker and assembly *(the white plastic and captive spring come out as one and should remain so).The white plastic is very easily damaged.*
*
So be careful with the striker assembly!! ( side note: If the plastic looks familiar, it's because it's the same type as used in household appliances like washing machines. for instance, in the little tab that shuts off the washer when you open the lid.) It's very fragile!*
Now with the barrel, recoil spring and striker assembly out of the slide, there is much less to worry about on the removal of the rear sight.
Turn the slide up right. Okay, this is important, first take a pencil or fine point silver Sharpie and mark the center of the sight front and back(so you'll have a reference point in the center for installation of the new sight.
*Unscrew the set screw ( the small Allen screw in the front of the rear sight) unscrew it, until it is out and off the rear sight completely. (this way there will be no way it can damage the striker safety as the sight is moved left to right & off the slide.)*
There is only one direction the sights are to be removed from the slide: left to right
*(IE: from the "M&P" monogrammed side to the ejection port side!)*
To keep from marring up the sight and the slide get the proper "gunsmith"type hammer with one side brass, one side nylon, like this one, the kind I use at work:
Okay, get a block of wood, lay the slide on its ejection port side down, and the rear sight end hanging off the end of the block of wood. Give good forceful taps with the hammer's nylon side, give good smacks but not any "Hammer of Thor" silliness. The sight will come off the right(ejection port) side. The sight holds the striker safety and spring in place, this is on the top-right portion of the slide under the sight. When it does the striker safety is under moderate spring pressure. It and the spring will come out, so be prepared and roll up a towel and place it where it will catch the safety and spring.
Okay, now to install the new sight. Remember the only way the sight should be returned to the slide, is this time, from right to left.
Since it will be an adjustable rear, you'll need to get a wooden dowel approx. the diameter of a #2 pencil out to a Husky pencil (If you don't know what a Husky pencil, then you didn't grow up in the 1970's! HAHA) This dowel is what you make contact with the base of the rear sight so you don't damage the adjustment assembly.
*(If this were just another fixed sight, you'd just smack it back into place w/ the nylon end of the hammer)*
Set the slide upright and place the striker safety and spring back in place and hold in place with thumb or finger.
*(by the way, no lube should be used in the safety channel, nor in the striker channel. those fragile plastic parts have all the lubricity that the channel needs. keep all things striker related as dry and clean as possible.)*
While holding the striker safety in place with one hand, push the new sight into place from right side(ejection port side) towards the left side with the fingers of your other hand. It should go on far enough with finger strength to hold the striker safety in place. ( However a quick tap from the hammer may be required just to be sure, if you so desire. One never knows about after-market compatibility)
Now lay the slide on it's left side atop the block of wood, and using the dowel as a wooden punch and placed at the base of the sight strike the dowel with the hammer until the sight is seated , centered on the slide. If the sight comes with a set screw, screw it back down in place. Your sight should now be installed!
Okay, return the striker assembly to the butthole of the slide. Holding the striker safety in, gently push in the striker assembly until it's in place. You'll feel it make contact with the safety with a "click".
The end of the striker assembly will stick up a wee bit and will prohibit the full installation of the back plate, but don't worry, just push it in with a pencil as you slide the back plate in place within it's guide slots you'll feel it pass over the edge of the striker assembly and when it does, you can remove the object holding it down as the back plate now has it trapped, continue to push the back plate in place until you feel it "click". The end of the striker assembly is now trapping
it in place. They work together under spring pressure to keep each other trapped.
All there is to be done now is to reassemble the pistol's main components and go sight it in!!
PS: When you perform this task, let me know in a PM and if you need it, I can give you my number, as a "phone a friend" resource, should you need a talk and walk through while working.
Best of luck!