With modern cars with electronic ignition, ignition timing shouldn’t change. But in older cars with breaker-point ignition, it can change. Regardless, if you check your car’s ignition timing with a timing light and find that it must be adjusted and changed to be brought back to spec, the way you adjust ignition timing is by loosening the distributor’s hold-down bolt, then rotating the distributor.
A handy way to rotate the distributor is to grasp the distributor’s vacuum advance. Move the distributor in one direction – say, counterclockwise. Watch the timing marks with your timing light as you do so. If they start to line up at the shop-manual specification, you are rotating the distributor in the correct direction.
If the timing marks get further apart, you are rotating the distributor in the wrong direction. Rotate the distributor in the opposite direction. Turn the distributor until the marks line up, then tighten the hold-down bolt to hold the distributor in place.
Recheck the timing marks with your timing light to be sure they didn’t change (in case the distributor moved a little while you were tightening the hold-down bolt). Ignition timing should be set with the engine idling at the manufacturer’s recommended speed (RPM). Your car’s service manual will give you specifics for checking and adjusting your car’s ignition timing.