You can remove the selector on a standard AR15 without removing grips, etc.

Place the lever half way between SAFE and SEMI (or however your receiver is marked) and punch it out. The detent should be captured; nothing to spring out.

"Replacement is the reverse" as the saying goes but you'll have to depress the detent with a blade or something.

You can't reverse a standard selector so the MP15-22 part much be unique and have the detent slots on both edges.

-- Chuck