If you want to use the Ethereum testnets Goerli and Sepolia, or previously added testnets for other networks, flick the 'Show test networks' toggle accessible via the network selector:
Alternatively, on Extension you can also configure whether testnets are displayed by going to Settings > Advanced:
Once this setting is turned on, switch to a testnet by heading to the network selector at the top of the homepage and scrolling until you find the one you need:
