How do you change the direction of rotation of synchronous motor?

A synchronous motor aways runs at the synchronous speed, which is the supply frequency divided by the number of pairs of poles in the motor.

For a 4-pole motor the speed is 1500 rpm (50 Hz supply) or 1800 rpm (60 Hz supply).

The mechanical phase of the rotor is liable to be a little unstable under changing load conditions.