This backflow will either need to be repaired or replaced.
It is possible to extract a broken test cock from a backflow assembly with an extractor. This backflow still may need to be replaced if the assembly fails the test. It is a good time to replace the Backflow Assembly with a model that has a replaceable seat for the check valve. Those models include the Conbraco/Apollo PVB4A and the Watts 800M4.