KP-15 Complete receivers ship with selectors that have the detent track ramped so they can be removed easily.

You may need to take the fire control pins out first so that the selector does not get blocked by the trigger bar.

Place the selector half way between safe and fire and push the selector out towards the left from the right side.

"HELP I INSTALLED A SELECTOR WITHOUT A RAMP!"

You're going to have to use a drill, probably a drill press to drill through the selector to access the detent and push it down with a punch or similar tool.

Mil-Spec selectors are made of hardened steel and you'll definitely need to have a gunsmith to it with a carbide bit or endmill in a drill press or mill.

Aftermarket selectors are often made of mild steel and it is possible to drill them out with a hand drill and the receiver in an action block in a vise on a work bench.  Take care not to lose control of the drill as it breaks through into the selector detent.  It is possible for the bit to snag at that transition and the drill to walk and damage the lower.

Worst case scenario if you damage the selector wall removing the selector, it is possible to re-sleeve the selector hole with steel or brass tubing by drilling the hole larger and pressing the section of tubing in.  the inside diameter of the tubing should be 0.125 to accomodate the selector detent.