I have the same issue and have been trying to Uninstall the update since yesterday (received error that it could not install but it did install). Have a bit more experience with Windows and have tried several actions - including most of those listed in the replies to Logan_23
Unable to boot to Safe mode - have no idea why that is happening
Task Manager shows Antimalware Service Executable taking big chunk of memory. Microsoft Defender Antivirus Service under Executable cannot be ended. Have a good anti-Virus/anti-Malware program so Defender should not be running.
Opened Windows Security and Managed settings to run off all Microsoft Defender Antivirus settings - Windows turns it back on
Used msconfig unchecked anything related to Defender in the Services tab. When I click Apply the system rechecks them.
Attempting to end process or process tree for MsMpEng.exe or WinDefend gives message X The Operation could not be completed. Access is denied.
It seems more like this "fix" from MS is malware than anything of benefit to the user.