The Angular CLI requires a minimum Node.js version of either v12.14 or v14.15. [ng] Please update your Node.js version or visit https://nodejs.org/ for additional instructions. code example

Example: the angular cli requires a minimum node.js version of either v10.13 or v12.0

I would recommend you to remove your NodeJS installation completely and re-install NodeJS through NVM. This way you can easily switch between multiple NodeJS versions and the other benefit of that for each active NodeJS version you may able to install specific Angular CLI version.

e.g:

nvm install 11  
nvm install 12
nvm alias default 12  
nvm use 11  
npm install @angular/cli -g
Mac/Linux: https://github.com/nvm-sh/nvm
Windows: https://github.com/coreybutler/nvm-windows
Also see some details on Angular CLI version here

P.S
If you where using brew, to uninstall NodeJS follow this steps:

brew uninstall node; 
# or `brew uninstall --force node` which removes all versions
brew cleanup;
rm -f /usr/local/bin/npm /usr/local/lib/dtrace/node.d;
rm -rf ~/.npm /usr/local/bin/node;