Redeeming is possible at any time (if you minted): uGAS can be redeemed for the underlying collateral before and after expiry. If you redeem before expiry, you must redeem the exact amount of synths that you minted. Any synths you lost due to IL have to be bought back. Alternatively, you can redeem any number synths except a minimum balance of 5 synths which you can redeem after expiry.
Once you approve the EMP (ExpiringMultiParty) to redeem your uGAS for WETH, the “Approve EMP” button turns into a “Redeem” button. Click that button again to confirm the redeem process and receive your WETH!