To fix Windows update error code 0x80070643, start by running the built-in Windows Update Troubleshooter, which can automatically detect and resolve common issues. Next, ensure your system is fully updated by checking for any pending updates and restarting your computer. Corrupted system files can also trigger this error, so open Command Prompt as an administrator and run the SFC /scannow and DISM /Online /Cleanup-Image /RestoreHealth commands to repair them.