MajorGeeks published a quick guide about fixing an error with the Windows 10 Update assistant.

Windows 10 Update is Assistant Already Running

When running or starting the Windows Update Assistant, some users have reported a problem with the app restarting itself or force-installing Windows 10 while returning an error "Windows 10 Update Assistant is already running". Try these steps to fix the problem.

Windows 10 Update is Assistant Already Running - MajorGeeks