If the tilt has been incorrect for some time and head wear is now uneven as a result, probably not a good idea to just fix the tilt. The head(s) should be relapped or replaced and installed correctly.
Depending on the model, tilt can be adjusted using a jig, basically a squared piece of metal or plastic, referenced to the head base plate or the chassis plate.
I've sometimes done it by eye, referenced to the fixed tape guides or even the capstan shaft, which should be fairly true. On some machines I've used the technique of looking along the tape itself and watching for any slight twist in it from the guide to the head.
The classic error is to set the tilt wrong and then compensate for the resulting vertical tape skew by raising or lowering the head height. It looks fine but you get weak tape pressure on one edge, with the risk of dropouts, and long term uneven wear (keystoning). With the standard 3 screw adjustment, once you have tilt correct you need to avoid putting this adjustment out when you adjust height, by varying the two height screws by exactly the same amount.
Cassettes are easier as the tape path is standardised and so there's one jig fits any and every machine.
Whatever, if you do it yourself take time, be patient, use lots of light and if your eyesight isnt what it used to be, like mine, use magnifiers.
Cheers Tim