Go to the transactions list, right click the transaction that is stuck, and choose the "Abandon Transaction" option.
If that option is greyed out, then you must restart Urals Core with the -walletbroadcast=0 option and then you should be able to use "Abandon Transaction".

If the above two options fail for some reason, then you can start Urals Core with the -zapwallettxes option to clear all unconfirmed transactions from your wallet.

Once the transaction is either Abandoned or cleared from the wallet, you can simply go to the Send tab and send the URALS again but make sure that you include a sufficient transaction fee.