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.
are you sure you pressed volume up? are you trying to get to the recovery mode to perform a factory reset? according to the articles below you need to release all of the keys after the samsung logo appears. did you do this?
also i have a tab s2, but it is the nook version......and my battery life is terrible now. it is what i expect out of a tablet that came out in 2015. devices these days only last as long as their battery will give them most litium-ion battreries have a life of about 3-4 years.....so this device is way out of that range. if you are into dyi, you can go to https://www.ifixit.com/ for guides, tools and supplies on how to change your battery.