+1 to what Clownbike said. You need to determine if it is a cassette or a freewheel. I assumed cassette 'cause that is what you said. If it is a freewheel you will probably be limited to a 12T or 13T as the highest gear.
I know the Neos will handle a 30T as I have one on my bike. Lower gearing is easy though, just go to a smaller chainring up front.