My apple iPhone 6 Plus Not support UPI ayment

An iPhone 6 cannot be upgraded beyond 12.5.5, it doesn't have the hardware needed to run a newer iOS.

You will need a newer phone that can run a newer operating system if you want to use that feature.

Financial institutions keep up with newest security for your benefit. Some hardware cannot keep up with those security patches, as in this case. If I recall, your financial institution is slowly transitioning from any iPhone6 support.