If you already have a copy of the EX alpha, delete it.
Ok, I didn't had a copy before, so I guess I'll have to skip this one.
If you don't already have Beta 7.25 download and install it as normal.
Did it.
Copy your sourcemods/ex folder containing beta 7.25 to sourcemods/ex_alpha (so you can still play the beta version too).
This is where problems begin - since I didn't had alpha before, I don't have such folder. Tried to create one, and follow instruction - gave me the issue I described before.
Extract the 7.26 patch to your sourcemods folder ( they should overwrite files in the ex_alpha folder ).
Restart Steam.
You should now see both the Exterminatus Beta 7.0 and Exterminatus Alpha listed in your library.
This is ok, I was able to see both of them, but only Beta worked.
Please, don't tell me to follow instructions again, as I did it like 20 times ;d Could it come from lack of previous alpha versions/Source SDK 2007? <or was it 2005? the older one, the newer one I have works with beta but not alpha>