I don't think there is an automated way to change the physical phones options like that. Turning no PC Connectivity, and changing the password happen in the same local menu on the phone, which is then stored locally I assume in its VRAM.
To get to this menu on your 53XX phone sets: press your Blue Applications Key --> select settings --> look for PC Connectivity.
My guess is Mitel did it this way to add a physical layer of security for the user, so not just anybody can use any instance UCX, connect to any phone, control it and mess with buttons, etc.