UPI Not Support

You are not going to be able to use the UPI app because the developer of UPI does NOT support it on the iPhone 6 and the iPhone 6s. Contact the developer of that app and state your opinion to them.

Axel F.

See the pic below which clearly states you need iPhone 7 for UPI.

Every app has its own boundaries. Please contact the app developer --> How to contact an app developer - Apple Support