Unless it's changed in the last few years, the Ruger rear sight is a near-copy of the Colt Accro sight.
To replace the sight blade:
Unscrew and remove the windage screw.
Use your fingernail to pull the blade over to the LEFT side of the base, then insert a punch and use that to help to GENTLY push the sight over far enough that it can be rocked up, and out of the base.
Be careful not to allow the tiny spring to jump out and get lost.
When installing the new blade, be sure it's facing the right way.
Looking at rear of the blade, you'll see two small projections on the bottom of the blade.
The LEFT side projection is a rounded lobe.
The RIGHT side projection has a sharp corner.
Clean and lube everything.
Insert the tiny windage spring into the base and slide to the LEFT.
Insert the left side of the new blade into the base at a sharp angle and use it to push the spring over until the blade can be rocked down into the base.
Put a dab of stiff grease on the notched end of the windage screw.
Screw the windage screw in and test the windage adjustment to be sure it moves freely.
