Watch a video on caliper rebuild. Some people get away with extruding the pistons with no pads on as much as they can and clean them well with Q-Tips and brake fluid. Less mess and work. It depends on how good you bench, tools and skills are. Can be done with the caliper off the forks, still attached to the brake line.
Or- Try just taking the pistons out to clean very well. Then reassemble with a little red brake grease on them. Seat the pistons all the way in, match the two sides of the caliper, reattach the caliper to the forks, re-install the tryre and rotors insert new pads, and then inject new fluid. I have been using a large 50 cc syringe and a short piece of clear tubing to vacuum fluid from the reservoir at the caliper nipple. Then pump the handle until you get resistance. The key to this job is cleanliness. No contamination. Brake fluid damages paint but cleans very well. Don't contamine the pads or disc with brake fluid. My bike has no ABS. The ABS might require special procedures when refilling and bleeding.