Please expand your question. Version 127.0 is only a placeholder for an eventual release and is NOT listed as an official download for release, beta, or daily. It works, but there is NO UPGRADE path for it.

I installed ver. 127 because I thought it was the latest release. Now I can't go back to 115.12.1 without a problem and losing all my emails. How can I go back ?????

127 was never on the thunderbird.net site and would only have been available elsewhere. Yes, you can go back. I suggest doing a backup of the profile first, just for safety. assuming windows, you can do that safely by - exiting thunderbird - copying c:\users\<yourid>\appdata\roaming\thunderbird to somewhere else THEN, - download 115 from thunderbird.net and install over the top of 127. look then for a compatibilities.ini file in profile and delete it. Let me know how it goes.