Dump all your existing Homebrew packages:

brew bundle dump

If you’re using custom shell installed by Homebrew (ie. fish), change it back to bash temporarily:

chsh -s /bin/bash

You may also need to change this in System Preferences – Users & Groups (right click on your avatar and choose Advanced Options…)

Uninstall existing Homebrew (and all its packages):

curl -fsSL -o /tmp/uninstall.sh https://raw.githubusercontent.com/Homebrew/install/HEAD/uninstall.sh
/bin/bash /tmp/uninstall.sh --path=/usr/local