Your Sym moped's speed is restricted by a washer on the front pulley wheel of the moped's variator system. The washer restricts your Sym's speed by limiting contact between the variator belt and the front pulley. Removing this washer derestricts your Sym moped, allowing the belt to contact the front pulley and permitting your moped to travel faster. In order to remove the washer, you will need some mechanical know-how as well as a few basic tools.
Remove your Sym's kickstart from the side of the moped's variator case. Unscrew the bolt end of the pin that attaches the kickstart with a socket wrench and pull the pin out slightly. Pull the kickstart from the variator case.
Detach the plastic intake case by unscrewing the three bolts that hold the case in place with the socket wrench. This will give you better access to the bolts holding the lid of the variator case on.
Remove the lid of your Sym's variator case. The variator case is the metallic, cylindrical case to which your kickstart was attached. There are three bolts on the top of the case and three on the bottom. Unscrew these bolts with a standard socket wrench.
Unscrew the centre bolt of the front pulley wheel in the variator system with an impact wrench. If you don't have an impact wrench, you will need to secure the front pulley wheel in order to remove the bolt with a socket wrench. You can do this by wrapping the strap end of a strap wrench around the front pulley wheel and using the strap wrench as leverage as you unscrew the bolt.
Pull the washer off of the front pulley rod.
Reassemble your variator system back together, as well as the intake case and kickstart.
Consult your state's laws concerning the top speed of mopeds. In some states, mopeds and scooters that can travel over 30 MPH are actually considered small motorcycles, requiring their owners to get an M-class driver's license. If derestricting your Sym moped allows it to travel over 30 MPH, you may need to re-register it with the DMV.