It seems like needing a scanner to reset the brake caliper piston is a bit of a myth.
The electronic parking brake manually actuates the rear caliper pistons. This means that, once removed, you cannot simply press or rotate the piston back into place. While many say that an expensive scan tool is necessary to complete a simple brake job, it’s simply not true. All you need are some Torx sockets, which you should already have in your toolbox if you work on your own BMW.
With the caliper removed, carefully rotate it (without twisting or pulling the brake line or power cable) so that you have clear access to the back of the electric motor. There, you’ll find two T30 Torx bolts. Use a wire brush to clean any rust out of these bolts so that you don’t accidentally strip them, and make sure that the socket is securely inserted before attempting to loosen them.
Loosen those two bolts, and the motor should come right off. You can suspend it like you did with the caliper itself, or carefully disconnect the power cable and set it aside. Be gentle when removing the motor, sliding it back and forth to loosen it so that you don’t tear the O-ring seal inside.
With the electric motor removed, you’ll be able to see where the motor interfaces with the caliper. Using either a triple-square socket or a T45 Torx, you can slowly rotate the socket clockwise by hand to retract the electronic parking brake.
From here, you can use a standard piston compressor tool to close the piston. Re-install the electric motor, being careful not to tear or pinch the O-ring, and you can now complete your brake job in much the same way as you did up front. Remember to re-connect the electric motors if you disconnected them.
https://bimmerlife.com/2019/07/20/diy-brake-jobs-and-the-electronic-parking-brake/