c:\program files\notepad++\updater\gup.xml (or appropriate path for your installations) has an entry called InfoUrl:
<InfoUrl>https://notepad-plus-plus.org/update/getDownloadUrl.php</InfoUrl>If you change that URL to a site you control and that your users have access to, then that URL should return XML of the syntax:
<GUP> <NeedToBeUpdated>yes</NeedToBeUpdated> <Version>8.2.1</Version> <Location>https://github.com/notepad-plus-plus/notepad-plus-plus/releases/download/v8.2.1/npp.8.2.1.Installer.exe</Location> </GUP>The <Location> value in that response indicates where the updater should download the installer executable.