Step 1: Open Settings
Press Windows + I to open the Settings app. Navigate to System → Activation.
Step 2: Change Product Key
Click "Change product key" and enter your 25-character license key in the format XXXXX-XXXXX-XXXXX-XXXXX-XXXXX.
Step 3: Activate
Click Next and then Activate. Windows will verify the key with Microsoft's servers and activate your copy.
Troubleshooting
If you encounter error 0xC004F213, ensure you have an active internet connection. For error 0x803FA067, try using the phone activation method via Settings → Activation → Activate by phone.