Website: Top left menu (3 lines on top left) > Service Center > Change Phone Number. 

AppHome page → Menu (bottom right) → Mutual Funds →  Service Center →  Change Phone Number. 

Note: If you have not invested in Stocks & ETFs, then you will be on the Mutual Funds page by default.

Enter the new phone number and we will send you an OTP for verification. Verify it and submit the request. We will first update your dashboard, then also update the KYC and the mutual fund folios in which you have invested (if any).