Samourai is one of the only Bitcoin wallets to allow you to top-up the miner fee of transactions ever 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 transaction and balance screen.
Step 2 - Increase Fee
Once you have tapped the transaction you will be prompted to choose another option. Choose Boost TX Fee to begin the process. You will be asked to confirm the new miner fee.
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.