Hello -- I've had this bike for 50 years but never learned to shift gears -- always rode on flat terrain without pushing the speed much. However, now I'll be riding with a group of people who like to ride fast and I don't know how to shift gears to do this. The gears are on the frame of the bike, not on the handlebars. There is a left and right gear shift lever on the frame. Please tell me how I shift these gears. Thanks!