You can reserve the mint cabin by paying with a combination of cash and TrueBlue points if you wish to pay for your flight using TrueBlue points but don't have enough of them to do so. Use TrueBlue points to upgrade to the designated cabin class by doing the following actions:

• Visit the JetBlue Airlines website's TrueBlue accounts page and log in.
• Booking your flight is important. Check the flight schedule for your flight to see whether the "JetBlue Mint Upgrade" option is available.
• Spend the required cash or miles to complete your flight upgrade. Follow the procedures displayed on the screen to complete your request.
• Your email address will be used to send you a confirmation email from the airlines and the updated cabin class.
• Complete the check-in procedure before selecting your seats.
• Make the boarding pass to finish the process.

Source Url:- https://community.wongcw.com/blogs/307380/How-do-I-Upgrade-my-Seat-on-JetBlue-Airways