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:-