I have that same switch but on a TRV19 (this is in my museum ) however I think you've got the still picture thing wrong. Stills are taken with the button above this switch on top of the camera.
Anyway... it's a rotary switch and it looks like you can get to it once you remove the right hand panel or perhaps see what's up by 1st trying to remove the panel under the battery. Basically without a service manual ya gotta start by unscrewing stuff. Keep track of which screws go where too because while from the head of the screw they all look the same they are probably different lengths in more than one position.
Does the switch still click at all 3 stops or or is it just sort of floating? Rebuilding a broken switch like that is probably a PITA. You might be able to re-engage the contact by simply putting a spot of solder on the contact or pull the contact back into play with some tweezers but to do that my guess is you have to be able to take apart the switch. If it's an "easy fix" you're not going to know until you open up the camera and to be honest that camera is basically worthless at this point so if you think it's worth it my advice is dive in.