how to combine sections in canvas
The way to "combine sections" in Canvas is called "crosslisting", which is really moving the enrollments from one course section into a different course.
If your school allows instructors to crosslist, the instructions from Canvas are at
How do I cross-list a section in a course as an instructor?
You may want to check with the Help Desk at your school, though, since some schools have different mechanisms to handle merges and crosslisting. At my school, for instance, we have a special form to help instructors do merging/crosslisting, though it actually uses this same Canvas functionality in the background.
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