Do nothing. Leave that account as it is, and visit when you please, participate when you feel like it.
Having an account on an SE site doesn't mean you have some sort of duty in participating actively in it.
If you want to inform people that you're not active anymore on a given site, you could edit your "about me" profile section to explain that, but it isn't really necessary. People can see from your profile itself (latest questions, answers, comments, edits, ...) whether you're active or not. Feel free to leave a note though if it is important to you.
Now if you are a ♦ moderator, then things are a bit more complicated, and you should get in touch with the community moderators and your fellow site mods to see what (if anything) needs to be done.
And if you were a Stack Exchange employee, then you would need to talk to your boss or to HR, not Meta.SE :-)