Ask them to freeze or block your account / UPI ID right now. Every major bank has a 24x7 fraud line — see the full number list here if you don't have it saved.
This is a free, government-run helpline built specifically for fast reporting of financial fraud. The faster you call, the higher the chance your bank can still stop or reverse the transfer.
Go to the National Cyber Crime Reporting Portal and file under "Report Financial Fraud." Keep your acknowledgment number safe — you'll need it for follow-up with your bank.
The message, the call log, the UPI transaction ID, the app you were told to install. Scammers often delete their side of the conversation — capture it first.
Don't transfer more money to anyone promising to "reverse" or "unlock" your funds — that is always a second scam. Don't delete the scam messages. Don't uninstall any screen-sharing app until you've screenshotted it.
You can read about how recovery and chargebacks typically work, or bookmark the full helpline directory in case this happens to someone else in your family.