It's a crank-forward, freewheel-equipped cruiser. And the stock gearing will take you past 23 MPH. I suggest you leave well enough alone.
I suspect that even if you did get a higher ratio, you'd struggle with putting more power into the bike from the crank-forward position. And you'd need a lot of that to overcome the drag of your upright riding position.
If you insist, finding a rim-braked, 26" cassette wheel (that'd take an 11T smallest) off an old MTB would probably be the cheapest and easiest way to continue.