How do you mark things as complete?

I swear I've tried to ask around but never gotten a clear answer. There used to be a guide on here but it never actually gave detailed info on how you'd go about getting their status changed. Does anyone actually know? There are so many I've read that need to be changed to complete and have just been left as is. I'd be willing to put in the work or submit the changes if I only knew how.

Does anyone actually know?