First unload and clear the gun!
IIRC, the rear sight is in a dovetail slot(?). If it is, you need to get a brass or plastic drift and move the rear sight a little bit to the left by tapping it gently (the direct of the bullet strike).
It's always a good move to use a pencil and mark both the sight and the top of the barrel lightly with a little mark so you can gauge how far you moved the sight each time you tap it.
Remember any adjustment will be multiplied by the distance you are shooting at. It may seem just a hair off at an indoor 25 yard range and you find it's a foot or so off at 100 yards. OTOH if it's dead nuts on at 100 yards it will still be on at 25 or 50 yards.
Just tap it a little at a time, shoot a group, and see what the improvement is, then unload, clear and tap some more as needed.
Or you can take the lazy man's way out, like the rest of us, and buy an inexpensive Simmons 22 Mag scope for around $30 and just ignore the old iron sights.