My rooted nexus 7 wont accept the 4.3 update it says i do not have enough space, but i still hve 18G

[Joe Cairo]

I was ignoring it, but now some times my launcher wont load, and ill have to restart it 2 or 3 times. I was given this nexus 7 from the i.t. girl at fords. I know she knows her ****. It was set up about 2 years ago, by her for me. She now out of the country. This is really my first interactions with any tech. In 16 years, so i cant even accurately answer all questions, without the help of a walk through. Im not sure if this is the 2012 or not.