Hi Ho: Well, you have enough contradictory replies that this should keep you going. Disconnect negative cable first is correct. If you touch ground with the wrench nothing happens (since the terminal is connected to ground). Then if you should happen to touch ground with the positive terminal nothing happens since there is no complete circuit with the negative disconnected.
Practical ideas: wear old clothes so that holes from the battery acid won't be a big deal. Clean the wire terminals with a wire brush and dip them into a solution of water and baking soda to neutralize the acid, then wash them with clear water. You can also clean out the battery compartment with the same solution to neutralize the acid. Clean, dry battery wire terminals will help keep the contact resistance low. I always try to spray the terminals with battery terminal protectant after installation. If you don't have (or don't want to get) it, some use chassis grease to protect them from corrosion. A 6-pt socket of the correct size will help get the terminals loose. Penetrating oil may also help.
Make sure your health insurance is up-to-date so that you can get medical help from lifting the heavy batteries.. LOL