If you want to swap just the CRT (tube) you need to:
- unscrew the Torx-bolts at each corner (T-15? T-20? I can't remember, sorry),
- get the anode (the thing under the "suction cup" at the end of the thick red wire) disconnected... make every effort not to kill yourself with this step (search for details)
- gently pull the board straight back off of the neck of the tube
Repeat for the other tube, swap, reverse the procedure.
If you want to swap the whole video system, you only need to get the corner bolts and slide the video board straight back out. You can leave the neck board and the anode attached to the CRT.
It would probably be easier to just swap the logic boards at that point though
Be super careful with the CRTs themselves. They're heavy and their necks are fragile. Once you let the genie out it'll never show video again.