HOw to make a unipolar stepper motor bipolar?

You would have to rewind the motor with bifilar wire. That isn't really practical. Removing the magnetized rotor from some designs (SloSyn, for example) will weaken the magnets, thereby reducing the motor's torque. It may be possible to get at the windings by removing only one or both end bells. All in all, it makes more sense to use the motor as wound and drive it with one H bridge per phase.

Jerry

Make it stop taking it's meds?

Seriously, the unipolar configuration has a grounded center tap and drives each side of the coil from a single driver transistor. To operate as bipolar simply lift the center tap and connect each end to an H bridge.

Wow! What a goof! I saw "bipolar motor" and thought "bipolar driver". A bipolar motor uses unipolar drivers and vice versa. Sorry!

If the center taps of your windings aren't tied together, leave them open, and use the individual phase windings end to end. The motor's voltage rating will double and its current rating will halve.

There's nothing you can do if the center taps are brought out as a common lead. I would advise against trying to separate them internally unless you have a few spares to practice on.

Jerry