Historically? Shutter the police department and open a new one, putting people you can trust to do good things in the face of adversity (like cops who were pushed out of the previous department for whistleblowing or refusing to engage in malfeasance) in charge of setting up the new system.

This was the traditional FBI strategy for dealing with bad departments and it works well, while most other approaches don't. Getting good people to be cops is difficult while bad cops are calling the shots and actively working to keep them out.

Getting good folks in charge from the get go lets them develop appropriate methods of identifying and retaining good officers appropriate to their locality.