In a scenario I have a signed transaction from A to B and ready to broadcast. As C, I want to broadcast it but the part I am having hard time to understand is as the transaction is already signed and everything is settled how can I change the transaction fee I would like to pay? Can I pay extra from my wallet for example? Thanks.
If you have send your transaction but it does not appear on the blockchain yet it mean it is still in the mempool, the place where transaction are waiting to be taken by a miner.
Miner incentive are to take the transaction with the biggest fees first, so yes you can "double send" your transaction using the same element but with an increased fee, so that it has more chance to be mined fast.
You can check this question for more detail: