How to downgrade to another java version

Not possible unless you can get it from a backup. Either restore from a clone or from a TM dated from before the upgrade. Used to be possible to get a package from a free Developer account in downloads, but the last time I looked it was no longer there.

But you can try searching there, if you like. If it is, should be able to install it side by side with the current one.

https://developer.apple.com/programs/register/

OK but if you are to do it from a Time Machine backup, what files do you have replace exacty. Is it enough just to replace the "java" folder in the system Library?