If the progress of Windows Upgrade Assistant is stuck at 99% for more than four hours, here's something you can try. Note: Make sure you follow these steps before you restart your PC. Otherwise, the Media folder might be unavailable.
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Open File Explorer, type C:\$GetCurrent, and then press Enter.
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Copy and paste the Media folder to the desktop. If you don't see the folder, select View and make sure the check box next to Hidden items is selected.
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Restart your PC, open File Explorer, type C:\$GetCurrent in the address bar, and then press Enter.
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Copy and paste the Media folder from the desktop to C:\$GetCurrent.
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Open the Media folder and double-click Setup.
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Follow the instructions to start the upgrade. On the Get important updates screen, select Not right now, and then select Next.
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Follow the instructions to finish upgrading to Windows 10. After you're done, make sure to install available updates. Select the Start button, and then select Settings > Update & Security > Windows Update > Check for updates.
