I have a student that was in a section with another TA but is now in my section.
I have had a look around for the best advice and this thread How do I move students between sections? brings up exactly the thoughts which came to my mind.
I would recommend having a read of that (and contacting your Canvas admin first) as your school/institution may automate this in which they would prefer to fix things on their end.
If they are OK with it, you can follow Nicholas's last step which is 'Under People [in your course], you will be able to assign students to a section using the "Edit Sections" option under the gear associated with each student.'
Please let us know if you have any issues and we can gladly help further!
Hope that helps!
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