Simple balance control is the easiest solution. Play a track with a well-defined center image and adjust the control until it sounds like it's in the middle.

If you want to get more accurate, you can use individual L/R graphical EQ's to adjust for it by frequency - easier to do with something like 10 or 14 bands but you can go full ham and do 31 if you don't mind taking the time.

Begin by just using a center image vocal track and setting the balance control, this will correct for most of the imbalance overall. Then use a tone generator to start playing a mono tone at each individual frequency on the EQ, and while listening, use each L/R eq slider as an individual balance control for that band, moving one side up and the other side down, until it sounds like that frequency is centered. Keep going until you've balanced them all, then enjoy.