Hello,
Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.
Please edit the post that you want to change the canonical to and then head over to Rank Math metabox > Advanced tab as shown below:
Please let us know if we can offer additional help.
Looking forward to helping you. Thank you.
Hello,
We are super happy that this resolved your issue. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/?rate=5#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.