Clear the weapon.
Final step you place the weapon on safe.

Look inside the PG and see if you need a hex head or flat head screw driver to remove the PG screw.
Remove screw and lock washer
Removed PG.
The detent spring is inside the PG.
Turn the receiver so that the detent drops out. If it doesn't fall free, rotate the selector.
You can now pull out the selector.
All of the ambi I have seen involve a selector that is in two halves. You insert the left hand side, then the right hand is 'keyyed' to the left and attached, usually with a screw.
Insert the detent.
Insert spring into PG, put PG into place.
Reinstall PG screw w/lock washer.