The answer to 'how to unclog backflow preventer' is to first locate the backflow preventer, usually installed near the main water supply line. Unscrew the cap or cover on the backflow preventer and use a plumber's snake or auger to carefully clear any debris or buildup inside. Be sure to wear gloves and eye protection for safety. If the clog persists, you may need to disassemble the backflow preventer to fully clear the obstruction.
Backflow preventers should be inspected and tested annually to ensure they are functioning properly and prevent contaminated water from entering the main water supply. If you are uncomfortable performing this maintenance or the backflow preventer is damaged, it's best to call a licensed plumber to service it.