You replace them!
Your bike disc brakes may be stuck closed due to dirt or debris, a misaligned caliper, or a problem with the brake pads. To fix this issue, you can try cleaning the brake system, adjusting the caliper alignment, or replacing the brake pads if they are worn out. If the problem persists, it may be best to seek help from a professional bike mechanic.
You need to replace your brake caliper on the side that drags. The caliper has several parts (piston & slider) that corrode and get "stuck". The only way to fix the problem is to replace the caliper. If you try cleaning the parts, it will most likely stick again within a few months.
if the caliper is stuck u need to replace the caliper it is also a wise move to buy a matched pair of calipers, cheaper , and replace both sides at same time
just replace the whole caliper.
Replace
Sticky/stuck caliper slides, sticky/stuck caliper piston, collapsed brake hose, pinched brake line,
A stuck caliper piston. Stuck caliper slide pins. Pinched brake line/hose. Frozen/stuck parking brake cable/mechanism.
If the caliper is sticking or leaking it is possible to buy a rebuild kit and replace all parts that are worn inside. If the caliper has a cracked or broken housing then the unit can not be repaired and must be replaced.
You can find detailed instructions on fixing Stuck brake sliders on youtube.com/watch?v=KpNlOnZiiFE
They can be rebuilt but not by a novice. Replace it with a rebuilt unit.
brake caliper get stuck ed
stuck parking brake/stuck brake caliper