How To Change Networks On MetaMask

To open MetaMask you simply need to click its icon. by default you will be connected to Ethereum Mainnet, but if you want to change it you need to press on the icon and choose the network you want to switch to. 

Chrome extensions
MetaMask Change network.png

After adding MetaMask extensions to Chrome, if you don't see it you need to press the extensions icon and pin the MetaMask to the bar.