So, for most characters you need to right click any part of their body and in the context menu you should see 'Open Power Menu' and if you click on that you should see UI of a human and if you click on one of the body parts you will see the powers that are attached to that body part

And then just Disable or Enable them, and to Activate it just do F, or if it says 'Alt Activation' Thats 'H' Hope it helps.

tell me anything else you don't understand

Also if you don't understand how a power works, there should be a description for it at the bottom Right of the UI