How to change to 5G wireless on my MACBOOK PRO, Mid 2012

"5G" is a cellular phone protocol, it is not natively available on any MacBook Pro.

5GHz band of Wi-Fi is available and useful, and faster and less crowded than 2.4GHz. But you need a modern Router to broadcast on that band. Your MacBook Pro 2012 is ready, willing and able to respond on 5GHz if you have a modern Router.

You can replace your antique Router in most cases, or you can ADD a new Router to many existing Routers. If you give them both the same network-name and password, they combine into one Network.