Samourai is one of the only Bitcoin wallets to allow you to top-up the miner fee of transactions even after you have already sent them. This can be useful if you sent the transaction with a low custom fee resulting in a very long confirmation time.
Step 1 - Tap the unconfirmed transaction
To top up the transaction fee of the sent transaction simply tap the unconfirmed transaction in the Main screen in Samourai. You will be taken to the Transaction Details screen.
Step 2 - Increase Fee
On the bottom of the Transaction Details screen, tap the button labeled Boost Transaction Fee to begin the process. You will be asked to confirm the new miner fee before the boost is attempted.
Step 3 - Wait for confirmation
Once you have sent the boost you will see a small transaction leave your wallet, this is the boost transaction. Both this boost transaction and your original stuck transaction should confirm within the next block.