From the dropdown list, select “Add Network” and enter the following:
Network Name: Mumbai(Polygon Testnet)
New RPC URL: https://rpc-mumbai.matic.today
Chain ID: 80001
Currency Symbol (Optional): MATIC
Block Explorer URL (Optional): https://mumbai.polygonscan.com/
Click “Save”, and you should now see "Mumbai (Polygon Testnet)" in the network dropdown.