In Blender, with the avatar mesh selected you go to Edit Mode (hit Tab key). Then select the vertices/lines/faces to be separated and hit "P" key. You get a small menu where you choose "Selection" and Bam! The selected vertices/lines/faces are now their own separate mesh. Hit Tab again to leave edit mode.
ok i have done it so what do i need to do with the arms and legs?170 days ago(+1)
What menu?
You upload the avatar with the suit, without the limbs. As for making some kind of system that lets you hide/show them using a menu in game, that's technical. Coding. Can't help you there, I don't code.