Your Android SDK is missing, out of date or corrupted

To change the update channel for an existing install, proceed as follows:

1. Open the Preferences window by clicking File > Settings (on Mac, Android Studio > Preferences).
2. In the left panel, click Appearance & Behavior > System Settings > Updates.
3. Be sure that Automatically check for updates is checked, then select a channel from the drop-down list (see figure).
4. Click Apply or OK.

It's from Android Developers Guidelines

1. File > Setting > Appearance & Behavior > System Settings > Android SDK > [Edit] Button Click (Blue Color Link Button)

2. then, it will be show a Dialog [ SDK Setup ]

3. push [Next] button & [Finish] button

Good Luck ~~ ♪♬ (^.^)

Files -> setting -> Android SDK -> you can see the 'edit' that you can change the path of the Android SDK. -> click the download button