Windows Server 2016: Update stuck on Downloading (wuauserv not stopping)

Windows Updates were stuck on Downloading for an extended period, sometimes even for days, without ever progressing to the installation phase.

In this situation, the wuauserv process is often hung internally, it keeps the service in a RUNNING state but does not progress the update workflow. Restarting the service forces Windows Update to reinitialize its session and re-evaluate the update state.

First attempt, normal restart:

Microsoft Windows [Version 10.0.14393]
(c) 2016 Microsoft Corporation. All rights reserved.

C:\Windows\system32>net stop wuauserv
The Windows Update service is stopping.
The Windows Update service could not be stopped.

Service state check:

C:\Windows\system32>sc queryex wuauserv

SERVICE_NAME: wuauserv
        TYPE               : 20  WIN32_SHARE_PROCESS
        STATE              : 4  RUNNING
                                (STOPPABLE, NOT_PAUSABLE, ACCEPTS_SHUTDOWN)
        WIN32_EXIT_CODE    : 0  (0x0)
        SERVICE_EXIT_CODE  : 0  (0x0)
        CHECKPOINT         : 0x0
        WAIT_HINT          : 0x0
        PID                : 1536
        FLAGS              :

The service was RUNNING but unresponsive.

Fix

Get the PID (in my example, it was 1536) and force terminate the process:

C:\Windows\system32>taskkill /f /pid 1536
SUCCESS: The process with PID 1536 has been terminated.

C:\Windows\system32>

After terminating the process, Windows automatically recreated the service instance.
Update status immediately moved from Downloading to Preparing / Installing.

No server reboot was required.

Short explanation: if Windows Update is stuck on Downloading, the service may be logically hung. Forcing a restart of wuauserv resets the update engine without rebooting the server.