i would like to restrict users form accessing my site completely for a few hours while i do a software upgrade. i plan to do this in the middle of the night so fewer users are affected. my question though: is there an option in cpanel to do this easily? recommendations? thanks!

You can setup a password on your main directory by clicking on the icon under security called "password protect directories". When you come to the list of directories click on public_html and fill in the required info. When you want to lift the protection you can remove the password. You can also name the directory something like "Site is currently undergoing maintenance.. Will be back up by so and so.."
This is the only way I could think of just by taking a glance at the control panel options. Someone may have a better solution.

Use the Redirects option and set a Temporary Redirect from your publicly accessible directories to a set file or directory explaining why the site is not as the visitor expected, and when it is expected to be back up.