If an unconfirmed transaction stays unconfirmed for too much and if no one rebroadcasts it, it will be "forgotten". It'll take usually 3 days to be "forgotten". You can resend it then.
You can also double spend it with a higher fee.
Bitcoin Core
Bitcoin Core makes making Full RBF transactions very easy. Simply 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 go to the Bitcoin Core datadir and delete the mempool.dat file. Then restart Bitcoin 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 Bitcoin 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 Bitcoin again but make sure that you include a sufficient transaction fee.
If none of these works, you can use a Transaction Accelerator.