In order to retrieve $sMAXI out of $wsMAXI, you need to unwrap your $wsMAXI. Just like for wrapping, you want to be on the wrap page to perform that action. Then click on the "Unwrap" button below the conversion rate.
Enter the amount of wsMAXI you wish to unwrap, or click Max.
Then, click on the "Unwrap wsMAXI" button and confirm the transaction in your wallet. You should now see sMAXI in your wallet, and no more wsMAXI!