Usually there's an auto-backup so you can rollback to the previous installed release without losing your config.
Assuming you did multiple 200.15 upgrades, so that's not working for you...
You
can use the
online tool here to decrypt your 200.15 backup, ungzip, extract the manifest file, edit the version, add your modified manifest, gzip, then re-encrypt
with the python/powershell scripts.
BUT DON'T, unless you have a bazillion funky rules or dpsk entries you mustn't lose. There are configuration migrations which run after every upgrade, which will have modified your config file structure. I haven't looked at the 200.15 migration (they're compiled executables, so I need to trawl through the decompiled code in ghidra), but there are usually enough changes so I'd be wary about running with an incompatible config.