If you have no bms, then use any cheap 50w RC charger to balance. hook up to one group of cells at a time, set the charger to 1s, and charge that cell group. You do not need to charge the full groups, just the ones lower than 4.15v. Over time, all your cells will lose ability to hold 4.2v for long, so inevitably some of your capacity does diminish after a lot of use. So don't sweat trying to get cells to 4.2v, that won't hold them anymore anyway. Just get them all the same, whatever that is.
If you have a bms, do this. Charge, unplug, let sit half an hour for the bms to discharge any high charged cell groups, then charge again. If the charger won't restart, ride around the block and then charge, let sit, then charge. Repeat till your bms has balanced the pack.