Here is the situation: I have a 28BYJ-48 motor, a cheap unipolar stepper motor. However, I would like to drive it as a bipolar stepper motor.

A 28BYJ-48 stepper motor

Unipolar stepper motors have 5 wires, whereas bipolar ones have only 4 wires. The 5th wire is a common wire joining the coils, creating 4 half-coils that can be enabled independently.

Schema of unipolar and bipolar stepper motors

Both types of stepper motors are not driven the same way. Unipolar steppers are simpler to drive since you don't need to reverse the current: the driver only applies the tension to the common wire and sequencially grounds the other wires to power the half-coils. The drawback of course is that they are less powerful because only half of each coil is powered at a given time, so you get half the torque for the same coil length.

Luckily, you can use a unipolar stepper as a bipolar one with twice the torque, provided you have a circuit able to drive it. The modification is indeed rather simple: you need to remove the common wire and cut the link between the coils.

Conversion of the unipolar stepper motor to bipolar

In pratice, on the 28BYJ-48 motor, the modification is simple to perform. The first step is to remove the plastic cover.

Cache removed from the 28BYJ-48 motor

Once the cover is removed, you can cut out the common wire at the center (red) and the link by scratching the corresponding track on the board with a cutter.

Common wire removed and link cut on the board

Last step, you should invert the two middle wires in the connector (yellow and pink). This allows to match the pinout of dual motor drivers boards like the TB6612FNG (a dual DC motor driver consists of two H-bridges and can also drive a single bipolar stepper motor).

Inverting the two middle wires in the connector

And voilà, you now have a bipolar stepper motor!