In the CAV-15 Receiver, the selector spring and detent load from the top and the selector is then slid over top of them. The selector does not need to be removed to service or install Mil-Spec Fire control parts. In the case of some match trigger cassettes like those made by KE Arms, the selector does not need to be removed for installation either.
If you do need to remove the selector doing so can be tricky for inexperienced people. If the spring is not trimmed appropriately, it can be difficult to impossible to remove the selector without breaking the inner wall. In some cases the track and the detent locations in the selector are too deep, also making removal much more difficult.
This article assumes you have broken the inner wall of your CAV-15 receiver in removing the selector. Repairing this to function properly is fortunately very easy.