Well thats the problem with downloading a pre-built build, you have no idea how it was put together, and the point of a pre-built is that it is what it is. If you wanted different games/emulators and such its easier just to build your own the way you want it in the first place.
As i said though you need to change it in RocketLauncherUI which is in the rocketlauncher folder, if i recall you highlite the platform on the left then click the emulator tab at the top and choose one. But its been a long time since i used it.
The maker of the build you have has a discord dedicated to his builds so that the best place to ask. This is the official Launchbox forums and we do not offer support to pre made builds for the reasons i stated in my previous reply.