How to upgrade unknown tablet

[AC Question]

I just bought a $15 Android Tablet from a friend of mine. tried to update the firmware by going to settings>about phone>software update but unfortunately this option is not enabled or it seems to be locked. Any ideas why this options is not enabled? Also the Google play store is not on the tablet seems to be disabled also. I tried everything reset and hard reset but it still goes back to its original state. What's weird about this tablet is there is no name like Samsung, Asus etc. The only model name I could find is Credit Slab, version is 4.4.2 Kernel version is 3.4.67. By the way I was able to enable the developer options. Please help I would like to upgrade it to a higher firmware. Thank you