Google don't seem to be able to locate a "Bolt wal mart bicycle" so it's pretty much guesswork from here. Odds are that you have a something called a freewheel on the back. These are pretty much all interchangeable, so you should be able to pick just about anyone with the same number of speeds.
Question is, why?
I repeat:Most bikes will happily let a decently fit rider hit 25 MPH / 40 KMH without any tweaks to the gearing. Are you really going that fast? for extended times? frequently?