At certain times after a cPanel update, you might come across a notification in the top right portion of your WHM, asking you either "You must reboot the server to apply kernel updates." or "You must reboot the server to apply software updates." or both.
To assist with identifying why you might require a reboot can be done with our internal WHM API Feature. You can run the following command via SSH or in the WHM Terminal:
# whmapi1 system_needs_reboot
cPs# whmapi1 system_needs_reboot
Above we can see the RPM's are listed for each service that might require new updates to the RPM. You can proceed with the reboot by selecting the notification in your WHM. And invoking a "Graceful Reboot" of your server.
Here is more information on how can update various parts of your cPanel & WHM server: https://docs.cpanel.net/knowledge-base/general-systems-administration/how-to-update-your-system/