Go to the Mint application on your phone. Make sure it is updated to the most current version of the Mint app. appsto.re/us/wBu5r.i
Login. Click the three lines icon in the top left hand corner of the app. Then, click on the Settings tab at the bottom of the page.
Enter a 4 digit passcode. It will asked you to input the passcode twice to confirm. Now click the home button.
Go back into the Mint.com application. It will ask you authenticate your finger to proceed. And you're done!