So you booted it into download mode. I'd be a little surprised if a device as old as the Tab S2 actually had a download available, so I suspect that is probably a spurious message and that there is nothing really downloading. But it would probably be safest to leave it for a while before you ignore what it says and reboot it, just in case.
But if you give it maybe a couple of hours nothing changes then at some point I think you'll have to just reboot it anyway, unless a Samsung expert here has some other idea.