it's a little unclear to me exactly what you are intending to buy and/or alter.

is it a flat bar bike with two chainrings or a flat bar bike with a single chainring? and i take it that whatever you buy you will want a 34t chainring?

anyway...

if the crank can accommodate a 34t chainring, it could be as simple as buying a compatible 34t chainring and installing it.

OTOH, if the crank can't accommodate a 34t chainring you may have to buy a new, compatible, if available (and here's where some research on your part comes in ) with your BB, crank that can, and a 34t chainring that is compatible with the new crank and install them both.

i don't see any reason why shifting would be an issue in either case, assuming the bike comes with a flat handlebar.