Type the following commands to uninstall the current version (taken from here):
sudo rm -rf /usr/local/sfdx
sudo rm -rf /usr/local/lib/sfdx
sudo rm -rf /usr/local/bin/sfdx
sudo rm -rf ~/.local/share/sfdx ~/.config/sfdx ~/.cache/sfdx
sudo rm -rf ~/Library/Caches/sfdx
Also,