I want to change the cframe of just one part thats welded without changing the other one's cframe but i cant do it because i dont know how to use cframe offset
Welds have a property called Enabled:
https://developer.roblox.com/en-us/api-reference/property/WeldConstraint/Enabled
If this doesn't work (it should), then just set one of the attachments in the weld to nil move it and afterward reattach it.