Synchronous motors run at a speed governed by the supply frequency. Therefore to change the speed, you must change the supply frequency.
How far do you want to change the speed, and are you stuck with the synchrounous motor? It is possible to change the speed, but is there an easier solution such as change the motor type?
For such a small load, you should be able to do run the motor from a a small amplifier and a step-up transformer, the amplifier being driven from a variable frequency oscillator circuit.