Reloading isn't really a thing in this system, which is kinda neat. For ballistic guns, the closest thing would be rolling a complication on your attack roll, though I'd be more likely to have a complication result in a jam, which you'd probably need two hands to fix.
Energy weapons have something more explicitly like reloading, in that a complication can result in the current energy cell/fusion core getting depleted, so you'd have to reload to deal with that complication.
I'd need to double check to see if there's a perk or something that'd specifically let you deal with complications with only one hand, but if not, I'd be tempted to say you could spend an action point (or give me one) to be able to do it.