Settling uGAS is only possible after expiry. Before expiry, you can just sell (or redeem if you minted) the uGAS token.
This will approve the EMP to spend your uGAS and return the WETH that you are owed. EMP stands for ExpiringMultiParty contract, UMA’s process of handling expiring synths.
After approving, click on “Settle” (same button as “Approve EMP to Settle”) to burn the uGAS and receive your WETH at the 30-day Median Gas Price. No input is necessary, the settle function will take all available uGas for specified month and return WETH. It will also close any open minted position and return remaining WETH thus successfully closing your position.