Clicking the button 'Sync Project With Gradle Files' should do the trick:
Tools -> Android -> Sync Project with Gradle Files
If that fails, try running 'Rebuild project':
Build -> Rebuild Project

Clicking the button 'Sync Project With Gradle Files' should do the trick:
`Tools -> Android -> Sync Project with Gradle Files`
If that fails, try running 'Rebuild project':
`Build -> Rebuild Project`
**EDIT**
Starting with [Android Studio 3.1][1], you should go to:
`File -> Sync Project with Gradle Files`

[1]: https://developer.android.com/studio/releases/index.html#3-1-0