the operating system on your device has been modified in an unauthorized way? While trying to update my device I get: 

"the operating system on your device has been modified in an unauthorized way"

I get this message. My device is also rooted. What should I do?

Galaxy Tab A (SM-P550)
As explained in the comments, the updater checks that the system partition matches what it expects (i.e. that the checksum is of a previous version of the system). Rooting the phone changes the system partition, so that it no longer matches. The updater detects this and fails, to avoid possibly bricking your phone by leaving it in an inconsistent state.

You have two options:-


Don't install the update.
Download a stock ROM for the phone (either the old version you already have, or the new version you're trying to update to, and flash it to the phone from a PC. You can get Samsung stock ROMs from SamMobile, and you use a tool called Odin to flash the image.
I know I said two options, but this is really the same as option 2. Instead of flashing a stock ROM, if you still want your phone to be rooted, you can look for an already-rooted almost-stock ROM, and flash that to your phone with Odin the same way. The XDA Developers often has such ROMs, so you can search there for the right version for your phone. See also Where can I find stock or custom ROMs for my Android device?