It's a good practice to link your Grab account to either Google or Facebook. This way, you can always have access to your Grab account even if you've changed to a new mobile number.
Access the menu () and tap on your name.
Choose to link your account to Google or Facebook.
You will be re-directed to Google or Facebook login page. Authorise the link when prompted by Google or Facebook.
You can unlink anytime by tapping Unlink from Facebook or Unlink from Google.
Tap UNLINK to proceed.