I've just made my first edit to OpenStreetMap! I marked a restaurant in my neighborhood in Portland, Oregon as closed. It's been closed for several weeks. I did that by removing the tags "name" and "restaurant" and "cuisine". Hopefully that's the right way to go about it!
Viewing the history of the area, it looks like either my comment got clipped, or I spaced out and forgot to type the whole thing. Maybe trackpad caused me to highlight and type over. Oops. Not sure how to edit the comment, if that's possible. If I could have done anything else better, I'd love for somebody to let me know.