Tablet update OS / change firmware

My head hurts, I give up I need help.

I have a tablet made by Grundig - big company but not for tablets. Problem it doesn't boot anymore it shows the Logo for hours and that's it. It's an Android System 4.2. I can access that menu and already did a hard reset.

I thought maybe lets update android before I try to get rid of the firmware.

In the end I mostly have the problem that everything is specific for exactly a certain tablet or phone - mine not included.
Is it really that hard? Do I look at the wrong places?