Hi @mrozric73 ,
Here’s a [https://www.youtube.com/watch?v=bCDqFNzQwWg|video] that may help.
It doesn’t specify the exact T-Touch model so hopefully it is also relevant to your model.
Once you have opened the watch take note of the orientation of the original battery (e.g. +ve side is marked on the battery) before removal so that the replacement is inserted the same way.
The battery type number can be found on the battery itself when getting a new one.
ifixit has a range of [https://www.ifixit.com/Store/Tools/Prying_and_Opening|pry tools] that may be safer to use and also cause less damage to the watch than using a blade as shown in the video.
Any waterproofing may be lost after the watch has been opened. Since Tissot did it before then presumably it would have been made waterproof again but maybe not. Just a thought
Hopefully this is of some help.