You can try these commands to install and change global Ruby version to 2.7.5

$ brew update
$ brew install ruby-build
$ brew install rbenv

$ rbenv install 2.7.5
$ rbenv global 2.7.5
After that, you need to export some configurations to define rbenv as default global ruby:

echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.zshrc
echo 'eval "$(rbenv init -)"' >> ~/.zshrc