I did this in Windows 10:
    1. Open Control Panel
    2. Then go to System and Security
    3. Then go to System
    4. Then go to Advanced system settings
    5. Then go to Change Environment Variables of the User
    6. Then click create a new environment variables
    7. Create a new variable named ANDROID_AVD_HOME
    8. Set its value to your Android directory, like D:\Android\AVD