In registry editor, delete the key and all its subkeys:
Now you would be able to select the sublime from open with.
you have ST2 and ST3
when I double click a txt file, I would like it opens
automatically with ST3, like it used to do with ST2.
Unfortunately, when I
try to change "open with" for txt files, ST3 is not on the list and if I
manually select the exe file nothing happens.