I think that I've seen how can this be done with Electrum, but search (even google) didn't find that post. That would have been fast.
So all advice I can give you is for Bitcoin Core and that means downloading the whole blockchain.
Import the key/wallet into Bitcoin Core then restart it with -zapwallettxes parameter.
https://bitcointalk.org/index.php?topic=1652574.msg16603766#msg16603766Then you can resend with higher fee.
Try to contact DannyHamilton or achow101, they seem to know much better about this.