Thanks for answering, but I don't have a forum on my site at the moment. However, the users can have their own pages where they upload a photo of themselves and, of course, change it. In fact I solved this problem by doing everything on one page.

Good news! I found this: https://support.wix.com/en/article/corvid-tutorial-creating-a-form-for-editing-data-in-a-database-collection The video doesn't help, but the instructions do. The only remaining problem is how to access a particular item to be updated rather than the entire database. I guess that can be done with filters.