It's odd that the Google Drive mobile apps for Android and iOS didn't allow you to edit spreadsheets, but Google has rectified that today with the latest version of the apps. Users can now edit existing cells as well as create new spreadsheets from their Android or iOS device. You can also see the edits made by other users in real time, and the update also lets users add shortcuts for specific Drive documents on your Android home screen. Head to the Google Play Store or iTunes App store for the updated application.
Source: Google Drive Blog