It's funny you should bring this up because i know someone else who also asked this same question once upon a time: "Should Revolutionaries Work in Reactionary Trade Unions?". Of course the conditions today are very different so you cannot simply transfer their conclusions one to one to your situation, but it helps us understand how a revolutionary ought to think about and discuss these sorts of questions.

Study your conditions, analyze them and apply the dialectical materialist method and draw your own conclusions.

If there are enough of a movement to remove the police union to warrant a reaction then there must be something to build on. I guess you talk to other union members about the reactionary nature of police unions to grow that movement. I don't have experience here though so take this with a grain of salt.

Learn socialist history and theory in regards to political struggle, figure out if you are good at propaganda or agitation or find comrades that are.

Do what you can to teach others that unions can do more than just manage the worker/employer relationship and that it has a moral duty to use its massive body to act. Don't expect to fix anything on your own, but every little thing you can do paves the way for the next person who feels the same way you do.

unfortunately not a lot of easy answers here. if you're not attached to your job you could quit and find a job with a better union, or you could try and rally the support of your fellow workers to get a better position within the union, but that could be very tough

honestly I'd just give up on the union and dedicate more organizing time/efforts outside of work then