Went backwards in plan and accidentally wore tray #9 when I should have been wearing tray #13. What should I do now?

I am understanding you wore #9 instead of #13? If so, you should contact your dentist and get their advice. If you have worn 10 through 12 successfully then you could place 10 thru 12 and see how they feel and fit. Where which ever does not fit well for about a week then move to the next aligner until you get back on track with 13. Hope this helps.

Oops! I've had patients do this. Dangerous. Better ask your dentist. Often you can try placing #13 but you need to be careful not to over stress the surrounding bone and the roots of the teeth by moving the teeth too far, too fast. You could damage the bone and need a bone graft, or the roots of the teeth might resorb and leave you with short roots. These outcomes are unlikely; however, they do happen; so best to fess up to your dentist and see what he recommends. Good luck!