In order to reset the stuck download of SCCM Service Update, first you need a copy of the update reset tool which comes within the SCCM download media.
Copy it on your SCCM server and from command line with admin privilege, run the following command:
[-S SQL Server FQDN of the top level site] [-D Database name] [-P Package Guid]
CMUpdateReset.exe –S server1.contoso.com –D CM_XYZ –P 61F16B3C-F1F6-4F9F-8647-2A524B0C802C
The package guid can be easily located from the SCCM console:
\Administration\Overview\Updates and Servicing
Right click on the columns and enable Package Guid.
After you run the above command, package will be deleted and you can restart the download by either restarting the SMS_EXECUTIVE service or Check for Updates in console.