How to tag an event on your Facebook photos?

I’ve got a few auto tag request on some photos from the event that I was attending but as I was going through my tag requests I wasn’t paying attention to one of them I just ignored all of them then I noticed that the one I went to also auto tag requested me but by then I have ignored the request already but now I want to tag the event back on the picture.
So how do you tag an event on a picture?