The answer to 'how to unclog backflow preventer' is to first turn off the water supply to the backflow preventer. Then, locate the access panel or cover and remove it to expose the internal components. Use a plumber's snake or other tool to clear any debris or blockages from the backflow preventer. Once the clog is cleared, reassemble the unit and turn the water back on, checking for any leaks.
Backflow preventers should be inspected and tested annually to ensure proper operation and prevent clogs. If you are unable to clear the clog or the backflow preventer is not functioning properly, it's best to call a professional plumber for assistance.