If you’re having trouble activating Windows 10, check out the below link on Microsoft Support website to troubleshoot and fix common activation errors in Windows.
Link: Get help with Windows activation errors
How to add/change Windows 10’s product key using Control Panel
- Use the Windows key + X keyboard shortcut to open the Power User menu and select System.
- Click the Change product key link under the Windows activation section.

- Type the 25-digit product key for the version of Windows 10 you want.
- Click Next to complete the process.

Second way:
- Open Settings.
- Click on Update & security.
- Click on Activation.
- Click the Change product key link.

- Type the 25-digit product key for the version of Windows 10 you want.
- Click Next to complete the process.