First go to your Metamask extension from the extensions menu:
On the top, you will see an indication of which network your wallet is connected to:
Click on “Ethereum Mainnet”, and then “Show/hide test networks”:
Turn on “Show test networks”:
Once “Show test networks” is on you will have access to more networks:
Choose “Goerli test network”:
Your MetaMask wallet is now set up to use the Goerli ethereum test network.