android phone. How do I change to English? 

Change the default language of your phone.  The app languages follows the phone.

1 hour ago, AAArdvark said:

Change the default language of your phone.  The app languages follows the phone.

Obviously, my phone is in English already.

1 hour ago, fred110 said:

Obviously, my phone is in English already.

I don't think the language can be changed in the app. Are there maybe more than one version of Grab in Play Store? Try uninstalling it and then reinstall it. 

Is everything in Thai or just some bits, like food menus?