WUAUCLT.exe

Windows Update Agent, download new Windows Update files.

Syntax
      WUAUCLT Options

Options:

   /a or /ResetAuthorization  Initiate an asynchronous background search for applicable updates.
                              If Automatic Updates are disabled, this has no effect.

   /r or /ReportNow           Send all queued reporting events to the server asynchronously.

   /DetectNow                 Initiate detection right away, Query the WSUS server immediately 
                              to see if any new updates are needed.

WSUS uses a cookie on client computers to store computer group membership when client-side targeting is used. By default this cookie expires an hour after WSUS creates it. If you are using client-side targeting and change group membership, use /ResetAuthorization /detectnow to expire the cookie, initiate detection, and have WSUS update computer group membership.

If Windows Update has been blocked, disabled or failing for a long period of time a long history of updates will build up in the archive, this can cause high CPU use. Resetting the client can help with this, but it will take a long time to run Windows Update afterwards.

Preventing Automatic reboots

In Windows 10, Run gpedit.msc

Navigate to: Computer Configuration ➞ Administrative Templates ➞ Windows Components ➞ Windows Update

Open this and change the "No auto-restart with logged on users for scheduled automatic updates installations" to ENABLED
In the control panel choose Update and Security > Windows Update and manually Click "Check for updates"

Alternatively set this in the registry:

HKLM:\Software\Policies\Microsoft\Windows\WindowsUpdate\AU
NoAutoRebootWithLoggedOnUsers (DWORD=1 to disable automatic reboots)

In Windows 11 the above will no longer work, the most you can do is set 'Working Hours' when Windows will not automatically reboot.

The PowerShell script below will set working hours to prevent any Auto Update Restart for the next 10 hours.
If this is set to run every 8 or 9 hours (3 or 4 times per day) via a scheduled task, then no automatic restarts will happen at all.

Do not completely disable reboots 24x7 as that will create a backlog of multiple incomplete system updates, I would recommend allowing a reboot to happen at least once per week.

$HrNow = (get-date).Hour
$StartHr = ($HrNow+23) % 24
$EndHr = ($HrNow+10) % 24
$key = 'HKLM\SOFTWARE\Microsoft\WindowsUpdate\UX\Settings'

Set-ItemProperty -path $key -name ActiveHoursStart -Type DWORD -value %StartHr% /f
Set-ItemProperty -path $key -name ActiveHoursEnd -Type DWORD -value %EndHr% /f

Fixing Windows Update when it gets stuck

Open Settings ➞ Troubleshoot Other problems ➞ Windows Update.

In Windows 11, this can be found in Settings ➞ Troubleshoot ➞ Other Troubleshooters.

If that doesn’t work, try deleting Windows Update’s cache by booting into Safe Mode, stopping the wuauserv service, and deleting the files in C:\Windows\Software\Distribution.

If all else fails, download updates manually using the WSUS Offline Update tool.

Examples

Asynchronous background search for applicable updates:

C:\> wuauclt /a /DetectNow

Send all queued reporting events to the server asynchronously:

C:\> wuauclt /r /DetectNow

“We must always change, renew, rejuvenate ourselves; otherwise we harden” ~ Johann Wolfgang von Goethe

Related commands

WUSA - Windows Update Standalone Installer.
wuauserv - The Windows Update Service
Windows Update Troubleshooting - completely reset the Windows Update client settings.
AskWoody.com - Blog with a focus on Windows Update conflicts.
%systemroot%\WindowsUpdate.log
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate