Excellent question and it turns out there's a whole article about the solution! https://www.esa.int/Enabling_Support/Operations/Rescuing_Int...

> By using a specially designed sequence of manoeuvres, the control team realised they could redistribute the angular momentum stored onboard the satellite using two different reaction wheels spinning in opposing directions, causing the spacecraft to flip.

> "So at this point we knew we could control the build up of energy absorbed from the Sun, and christened this new manoeuvre the z-flip".