Hello, I was wondering on how to take out used uranium fuel cells becouse I tried it with inserters and it wont work

I HIGHLY recommend the BurntExtractor mod for this as that what it does for nuclear reactor type fuels: "https://mods.factorio.com/mod/BurntExtractor"
Also re-balance update adding more uranium output pending ;)

So i installed that mod, "BurntExtractor" and it still wont take the uranium out of the MK9s. Is there something special im missing here? Like a certain building/inserter i build to empty the slot?

strange, will have to test it outside of sandbox mode... thats been my go to for used fuels...

you must craft/use the unique 'extractor inserter' in that mod.

that inserter is specifically coded/scripted to pickup that slot from any entity...