This plugin adds two commands to change the casing of output:

• tolower
• toupper

These are basically aliases for tr '[:upper:]' [:lower:].

echo "Foo" | tolower
# foo

echo "Foo" | toupper
# FOO

I recommend Antigen to install and manage your ZSH plugins.

Put the following in your ~/.zshrc:

antigen bundle rtuin/zsh-case