My faucet is leaking, but when I remove the handle, I can't see how to get the cartridge out.

It looks like this: https://www.flickr.com/gp/7865232@N08/Qc2r91

I tried using a wrench on the nut-shaped top of the metal housing, but it would not budge in either direction.

Am I missing something simple here?

Thanks.

-J