12 volt battery replacement

Disconnect the negative first and connect the negative last. Also it's helpful to have a 10mm socket and extension

 

A few tips:

1. Disconnect the negative cable where it bolts to the body, rather than trying to remove the cable connector at the 12V battery. Swap the cable to the new 12V battery while you have both batteries out of the car.
2. Remove the brake power supply which is located next to the 12V battery, for improved access. You will need 10 mm and 12 mm sockets for that. There is no need to remove the wiring harness connector leading to the power supply.
3. It is not necessary to totally remove the red plastic cover over the positive battery terminal. Just bend it up enough for access to the terminal nut.
4. When installing the new battery, first mount the traction battery duct that runs over the 12V battery before you tighten any other hardware. If you drop the screw that holds the duct in place, it is pretty easy to remove the duct, lift up the 12V battery, find the screw and try again. If you save this for the last step, then when you lose the screw you'll be tempted not to find it, which would be an example of poor workmanship.
5. You might avoid losing that screw by using Vaseline or a similar heavy grease in the socket to hold the screw in place.

 

Not for sure if it applies, but sometime one of those ratcheting 10 mm closed end wrenches help out.
Just make sure it's a well made one.