Touch ID allows users to store up to five fingerprints. In turn, any of the registered prints can be used to unlock your iPhone and authenticate purchases associated with your Apple ID, such as App Store purchases.
So what should you do ? Firstly go to Touch ID settings section by going to Settings—> General—>Touch ID & Passcode—> Touch ID. Once there, you’ll see a list of fingerprints under the fingerprints heading. You can have up to five prints assigned to Touch ID, but you don’t have to assign all five if you don’t want to.
Once there, simply touch the Touch ID sensor like you normally would when you want to make a purchase or unlock your phone. If the finger you used is currently one of the registered prints listed under the fingerprints section, you’ll see the a gray flash to signify the assigned print.
After identifying the fingers associated with the assigned prints in Touch ID’s settings, it’s best to assign names to the fingers for future identification. In order to do so, tap the edit button in the upper right-hand corner of the page, and this will place you into edit mode. You can then tap on the names of each finger, and assign your own custom name.