You should be able to increase the fee of your transaction by right-clicking on it in the "History" tab and selecting "Bump fee". Keep in mind that the extra fee will be deducted either from your change output or directly from the amount you sent if you don't have any change. If you are running the latest version of Electrum then you should also see "Cancel tx" which will automatically double-spend the transaction back to you with a much higher fee. The first option will be probably better. What fee does your transaction pay right now?