Your photo isn't clear, but it looks like you have the white speed restriction wires. if they're two single wires connected in a loop, disconnect them to release the speed limit.
If you're successful in releasing the software speed limit, the top speed will still be limited by the motor design and the current from the controller. 12 amps is very low, and not enough power to go much past 15 mph on the flat. The motor has a top speed related to the number of turns in the coils inside it. You can't change that. I'm going to guess that it will max out at around 19 mph, but the power will ramp down from 15 mph, so it'll only achieve that downhill.
If you want actual 20 mph riding, you'd need to change the battery and controller to 48v, but then your brakes might not be good enough. When I ran my 20" wheeled Dahon with 48v at that speed, the brakes wore out extremely quickly. If you're not too heavy, that might be OK, but not if you're say 90kg or more. 48v will give you approximately 30% increase in power, torque and speed. You can also go up to 15 amps, which will increase power and torque by another 25%.
I disagree with the above post. Oil cooling wouldn't be needed. I've run many 36v motors at 48v and only one showed signs of getting warmer tha normal, but it was already a fast winding and it had weak magnets or insufficient amount of copper in the windings. All the others didn't seem to get any warmer than when at 36v.
A decent 48v controller and LCD is about £80. A 48v battery can be had for about £200.
Your 36v battery can probably give a bit more power. You can increase the current from the controller to say around 16A by adjusting the shunt value inside it by adding a bit of solder. If you add a big blob of solder to 25% of its length, you'll get approximately 25% more power and torque, so it'll accelerate faster and hold a higher top speed at the cost of the battery going down quicker. Alternatively, you can buy a 15A controller with LCD from about £40. You shouln't go much more than that because your battery might not be able to provide that much current, so will start cutting out. 15A should easily be alright and a good channce of 17 amps, then cut-outs become increasingly more probable.
Your control system is not very good. If you increase the power, it'll make the start-up a bit jerky because on each od the three levels, you get the same maximum power but with different maximum speeds. That's where the £80 controller is a lot better. It's power algorithms are much more user-friendly. None of that matters when you only have low power, but it becomes more important when you increase the power.