Lift the steel battery tray out of the lower case and remove the trigger button/SD board assembly.

When removing the trigger button/SD board assembly, don't lose the two steel nuts (highlighted in red) inserted into the lower case.

If your trigger button happens to fall apart during diassembly, the third picture shows its correct installed orientation. Be sure the spring is installed in the position shown, and that the two trigger button covers are unique to the side they are supposed to be installed in.