Use a key you already legitimately possess for the installed Windows edition. This does not bypass activation or establish entitlement to another edition.
- After an update, an already licensed PC displayed the Activate Windows watermark.
- Connect to the Internet.
- Open Settings > System > Activation.
- Choose Change product key.
- Enter the existing valid Windows key and select Activate.
- Restart or revisit Activation to verify activated status.
Reported outcome: The original poster re-entered the existing key and explicitly confirmed that activation was restored.
Verify the edition and activated status in Settings. If the existing key is rejected, preserve the activation error and follow the licensing support path instead of trying unrelated keys.