Whatever u do, don't upgrade to 4.3 by official updates because you will lose warranty. If you want to keep it clean then this is what u need to do:
1. Install custom recovery ( I recommend. Philz's)
2. Flash Mj5 bootloader ( very important so you can keep your warranty)
3. Now you can do whatever u like. Flash custom roms etc.
I know it's not much explained but I'm sure someone who's got more time will explain better. This is just quick guide so you can stay safe.
Sent from my Nexus 7 using XDA Premium 4 mobile app
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