The gear ratio changes whenever you change the number of teeth. In your case, since we're talking about the front chainring:
More teeth: harder to start, more difficult up hills, but more speed (Fire Stingray)
Less teeth: easier to start, easier up hills, less top speed (Golden Fox)
What is the rear teeth count? According to the manufacturer's website, it's 16t freewheel / 15t fixed cog.
Let's say it's the 16 in that case, you have a 2.75 gear ratio. With this you can do the following speeds:
Cadence / Speed
10.8 mph@50 rpm
12.9 mph@60 rpm
15.1 mph]@70 rpm
17.2 mph@80 rpm
19.4 mph@90 rpm
21.5 mph@100 rpm
23.7 mph@110 rpm
25.8 mph@120 rpm
28 mph@130 rpm
(source: https://www.surplace.fr/ffgc/)
So that gives you an idea of what your current ratio can do and if you wish to change things up, it all depends on your geographical location and you own abilities/needs/wants.
As for replacing the cranks, most crank arms will allow you to take the chainring off in order to replace them. So all you really need is a new crankarm. They're not terribly expensive and not difficult to change as long as you have the right tools.
But you can't put just any crank arm because on a single speed/fixed gear, it's very important that the chainline be straight. You need to see what size your bottom bracket is first and buy a crank arm that conforms to that sized bracket, otherwise you have to replace the crank arms AND the bottom bracket.
If you're unsure, just let the shop take care of all those issues. Or find a bike co-op that can help you.
