To add to Chalo's summary, you can replace the battery, controller, and display with a higher voltage, current, and wattage setup to proportionally increase the speed, but it's likely that only the display will fit where the old one did, so you'll have to secure the new larger parts to the bike in other places.
At a guess it would probably be $500-$1000+ depending on how much battery you need to do the job you need the bike to do for you under the specific conditions it has to do that job, and for how much range you need under those conditions. The faster and farther you have to go and the less flat and windless the conditions are, and the less perfect the roads are, the bigger a battery (and controller, etc) you will need.
The motor may be unhappy with the extra load and heat generated by using it beyond it's original design, but it may handle it ok. If not you can replace it with a better one; the cost for that will depend on what you need, probably another $200+.
At that point, however, you've replaced everything except the bike itself (which you may also have to replace parts on to improve it to work well enough for you at the higher speeds, such as braking and/or suspension or wheels), and it would then be cheaper to return (or sell) that bike as it is now and instead buy or build a bike that already does what you want it to do.
If you still want to try increasing the speed, but prefer to do it in bits and pieces to just see what happens, the first part to replace is the controller and display. You'd need one with a higher current limit than the original (because it takes more power to go faster because of air resistance), and one that has no built in speed limit. The display usually has to come with the controller to work with it, as they are not typically intercompatible.
Depending on the connectors used on the bike, you may have to cut and splice wiring to change parts out, and test for or guess at the correct wiring as there aren't any standards truly adhered to in this stuff, and not even the colors of the wires are consistent, so you can't always just match colors (it's possible to blow up parts by connecting them wrong, so it's best to research and test first...but often enough you have to just guess at some things).
Once you start this process you probably can't return the bike and would likely have a hard time selling it if it isn't working, so be sure you want to and can afford to go down this path before you start.
