If it was sold as a whole bike, the controller may have firmware on it that internally limits the speed to make it legal for the company to sell in your locality; tha'ts not uncommon. If this is the case, you may have to replace the entire controller to remove this restriction. (it may also be programmable with the OpenSource Firmware by Casainho, Stancecoke, et. al; there are at least a couple of threads for that in the Motors/Controllers section). Note that it is possible to "brick" a controller during a firmware update.
If there are any wires soldered to the PCB that simply loop up from one pad and then go back to another, without connecting to anything else, they could be control / limiting wires, and cutting one of them might remove the limiter (but it could just make the controller nonfunctional until it is reconnected).
If you have a wheelsize setting in the display, you could change it to the largest (or maybe it's smallest; cant' remember ATM) wheelsize it will take. This will make the speed sensor think the wheel is spinning slower than it really is, so it could allow operation at a higher actual speed. (assuming it doesn't also monitor motor speed and limit using that separately). This will also make your speed display incorrect, however.
If there is no wheelsize setting in the display, and it uses a separate wheel speed sensor (rather than a sensor from the motor itself) you could add electronics between the speed sensor and the controller to reduce the number of pulses (divide by 2, etc) received by the controller from it, which will make it think it's going slower, same as cahnging the wheel size.
If you don't need any speed reading at all, you could remove the magnet for the speed sensor completely, and then it would think you were still stopped all the time, and not limit speed. However, some systems timeout in a minute or two with no speed signal, and shut off. If that happens on yours, you'd need to turn it back on every time this happens.
It's also possible your system simply can't go any faster. If you turn the bike upside down with the motor wheel offground, how fast does it "go" at full speed? If it's speed limited, it will be exactly the same as if you were riding. If it is not, it will go a little bit faster than when you're riding. In the latter case, you'd need either more voltage (so probably a different controller and battery), more current (also probalby a different controller and battery), a different motor that is wound for a higher speed at the voltage you already have, or some combination of all three.
