That's simple. You can link your mobile number to Tata Pay UPI by following these steps:
1. Login to Tata Neu
2. Click on Tata Pay
3. Click on ‘Link your phone number on Tata Pay UPI’ nudge
4. Follow the steps you see to link your mobile number to Tata Pay UPI

If your mobile number is already linked to some other UPI App, you can still follow the same steps to link your number to Tata Pay UPI