A) Acquire a new slide stop. Buy one or call Glock, I would assume Glock will mail you a new one.
Once you have it, it's very simple to replace. All you'll need is a punch or suitable tool to push the trigger/slide stop pin through on the frame.
A/B)Clear the weapon! (Safety first)
C)Remove the slide.
If you have a Glock model with two pins in the locking block, such as the 40 cals do, I would suggest you leave the top one in as the slide lock is held in by the bottom pin and your slide stop spring will rest BELOW the top pin. (If you have a 9mm or model with only one pin you have no worries)
D)Push the pin from Left to Right until it clears the slide stop- the slide stop is the last part on so it's the first one off the pin.
E)Take the new slide stop and push it in and align the holes- Re-insert the trigger pin. FWIW- you should not have to push the pin completely out for the change.
***The only problem that can be experienced is if you have a two pin model such as the 10's and 40's-The problem is that some folks will install the bottom pin first and then the top. By doing this- they'll then drive the top pin in and unless the slide stop spring is moved out of the way- the spring gets damaged as it is supposed to rest under the top pin. If the spring is not moved out of the way- it'll get damaged and the SS won't function properly OR the Slide Stop spring will rest ON TOP of the upper pin which causes the SS to malf as well as reversing the direction of the action of the spring which in effect constanlty pulls the SS up instead of down. (Hope that ain't to confusing)
F)Re-assemble, get to the range and Glock on!
Sly