How do I regulate voltage on a multi-amp battery charger. Answered

I know very little about electronics. I have a car battery charger that outputs in 12 voltsDC. It is also "switch" selectible for 2 or 10 or 50 Amps. Once I select one of these choices 2/10/50, I would like to vary the voltage up and down from 1v to 12v. How do I build something that will vary the voltage at each of these amperage levels. Thanks- Steve