Hi,
it depends on your hardware.
* a full bridge: swap the left side with the right side.
* single PWM with (0V / 3.3V levels) can´t generate negative values. Thus you need to add an offset on the digital side and subtract it on the analog side.
Subtracting an offset on the analog side could be done with a high pass filter, for example.
* single PWM with (-3.3V / +3.3V levels using an analog switch) just needs to add an offset on the digital side.
* ...
--> show us your hardware details and we can give more focussed answers.
Klaus