Well depends. Check the tablet's "Settings" and then, "Update" or "System Update" and see if trying to Update Firmware in there shows any updates. If it does, then the popup is for a legitimate update and it won't go away unless you do the update, or turn off the system notifications.
If, however, there is no update listed in "Settings", then this is not a legitimate update and you will need to check the device for malware, viruses, etc.