Mi Pay is not an app (like google pay, paytm). Mi payment solution is built into the MIUI 10 framework, and you will need a Xiaomi smartphone with MIUI 10 Global Beta ROM to try it.
To start using it, make sure you have compatible smartphone, like Poco F1, Redmi 6-series, Redmi Note 5 / Note 5 Pro, or Redmi Note 6 / Note 6 Pro. If you have compatible smartphone, you can head over to MIUI forums (en.miui.com/thread-4970588-1-1.html), and download the latest build.
Note: The page also has the flashing instructions, so follow them thoroughly and properly, as even a small mistake could potentially brick your smartphone. Proceed with caution.
Setting up Mi Pay on your Xiaomi smartphone
First install the MIUI 10 Global Beta ROM on the Redmi Note 6 Pro. Ensure that you have signed in to Mi account on your smartphone. Now, head over to Settings > Accounts > Mi Pay Account.
Tap on My Profile, and see if your details have been pulled (Mi account number and phone number). The third option here is Default Bank Account, tap on it, and tap on Add bank account.
You will see a list of banks that are a part of UPI. Tap on your preferred bank with which you have an account. The system will then look for your linked bank account, and fetch details. You will then be asked to enter your UPI PIN.
Note : If you ve forgotten, you do have an option for forgot UPI PIN to reset it.
In case you are setting UPI for the first time, you will have to enter the card number, expiry date and CVV number. Next, you will have to create a UPI ID – the VPA (virtual payment address). It could be your mobile number or name or so, followed by UPI address.