You dont you can change gears using the B button.
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heres only a bike no acro bike or mach bike
get a gameshark and get acro bike
there is no acro bike on Pokemon platinum, you press B to change gears
you need to go to Mauville to get a bike an Acro bike or a Match bike
Acro Bikes are only obtainable in Ruby, Sapphire, and Emerald.