How do you change your name on Zoom, both before and during a meeting?

Two places. Permanent: sign in at zoom.us > Profile > Edit next to your name > save, and it sticks for all future meetings. In the moment: while in a meeting, open Participants, hover your own name (tap it on mobile) > More > Rename. That only changes it for that meeting. If the host locked renaming you will not be able to, so fix it in your profile before the next call.

The mid-meeting Rename is per-meeting only, so to fix it for good do it in your account Profile on the website, not the app.

On mobile, tap your own video tile > Rename. Same thing, just tucked away compared to desktop.