I figured out how to add
set -g prefix ^B
I typed the same command at the tmux : line and indeed ^B is now my prefix character, but ^A is also still my prefix character. I'd like to unbind it. I've tried unbind ^A at the : prompt. I've tried unbind-key ^A and unbind C-a and unbind-key C-a and none of them work.
How can I unbind ^A without restarting my tmux session?