UPI is a payment platform that enables multiple bank accounts to be handled by single phone combining several banking features like money transfers and merchant payment under one umbrella. It acts as "peer to peer" payment platform which can be scheduled or paid as per requirement. It allows a customer to pay directly from a bank account to different merchants, both online and offline, without the hassle of typing credit card details, IFSC code, or net banking/wallet passwords.
There are currently 53 major banks are member of UPI (CITI bank is a notable exception). 34 banks have their own UPI app in Android Playstore.
UPI-PIN (UPI Personal Identification Number) is a 4-6 digit pass code you set during first time registration with this App .You have to enter this UPI-PIN to authorize all bank transactions. It works like ATM password which should not be shared with anyone.
In case someone forgets the MPIN, he needs to re-generate new PIN using his debit card details (last six digit of debit card & Expiry date).
In case of mobile loss, one needs to simply block his/her mobile number. Since, UPI-PIN (MPIN) is also required to perform any action, it is very safe.
In case of change in sim/mobile/application of the PSP, customer would require to re-register themselves for UPI after updating the same in all bank accounts.