Where can I get OH 2.5.0M1 for downgrade?
but I need openhabian version
Can I just install it via openhabian2.4?
There is no such thing as openHABian 2.4, it’s 1.4.1 or 1.5.
You can use the menu to change between latest stable, latest testing (milestone) and latest snapshot releases, but you cannot select any older version. If that’s what you want you need to download the package (see @rossko57’s post) and use your package manager or see here how to install older version from the repository.
I have used the command @mstormi posted about it works like a champ! Takes a few mins, just make sure to clear your cache.
Stop openHAB, then
Sudo openHAB-cli clean-cache