Two common options are magneto-torque rods that that use the earth's magnetic field or reaction wheels that spin masses up and down to provide torques to provide attitude control. They've documented their attitude systems btw on the site [1] it's magneto-torques and a "spin torquer" which I hadn't heard of but looks like it's another type of magnetic attitude control.

[1] https://upsat.gr/?page_id=26