How to download custom os?

You can download a custom ROM like cyanogenmod from here. You will firstly need to root the tablet and have custom recovery for your tablet in order to do this. Root isn't required if you can flash a custom recovery on your tablet but sometimes it's easier rooting it first then installing cyanogenmod via application or flashing it via the ODIN application.

How to root the phone (Yes you can 1-click do it with an application, but you will need to enable USB debugging mode under settings > development options (you hit build number multiple times in the about phone to enable development options)

Root application:

http://www.iroot.com/

Where to get cyanogenmod recovery:

http://forum.xda-developers.com/showthre...

Where to get the rom:

http://forum.xda-developers.com/showthre...