NinitePro.exe /updateonly /silent updates.txt
will update all Ninite supported apps on a machine to their latest versions and leave an activity report in updates.txt. This only updates apps and will not install any new apps.
This feature can also be combined with the /select flag, so if you just want to update some browsers you could run:
NinitePro.exe /select Chrome Firefox /updateonly /silent updates.txt
These features are only available in Ninite Pro.