Click and open the standalone SDK manager, then install/update the
Alternatively, you can try the standalone SDK Manager: Open the SDK manager and you should see a "Launch Standalone SDK manager" link somewhere at the bottom of the settings window.
Check your /platform-tools directory and if it does not exist, then open the SDK manager in the Android Studio (a button somewhere in the top menu, android logo with a down arrow), switch to SDK tools tab and and select/install the Android SDK Platform-tools.
The ADB is now located in the Android SDK platform-tools.