to change, to shift, to switch, to alter; to modify; to switch, to switch up, to change, to trade, to swap (usually + de) (when implying changing from one thing to another); to replace, to change out, to swap out; to exchange, to trade in
gear change, gearshift (a mechanism which changes gears in a vehicle); gear shift, shift lever, stick shift; (UK) gear lever, gear stick (the lever used to change gears in a motor car); derailleur (the mechanism on a bicycle used to move the chain from one sprocket (gear) to another)