I have Samsung Galaxy Nexus running on Android 4.2.1. When I run system update it outputs "Your system is up to date". How can I update Android to 4.3 EDIT: Device's model: Galaxy Nexus (maguro) Product code: yakjuxw I'm considering to install CyanogenMod. Can you recommend good backup software.
You have to wait until Samsung lets out their update with their added components, or flash a custom ROM onto it. I don't know how to do that, so I won't provide a recommendation. Depending on what network you're on, it may be a little while yet. An article about that.
Segmenting off of what Matt Voboril said, you can go over to XDA Developers and learn how to unlock your phone (so you can tinker with it), install a new recovery (so that you can manipulate at the system level (backups, side-loading, installing new roms, etc.), and install a ROM with the latest android (4.4.2 ATM). If you are using Verizon's gnex, go here. If you have the GSM varient, go here. I would recommend anything that ParanoidAndroid, AOKP or CyanogenMod have to offer.
As the others have said, you have to wait. But it would help to know which Galaxy Nexus you have. Is it a GSM one or Verizon's Galaxy Nexus? I'm pretty sure Verizon has given up on updating that phone at this point. And regardless of which version you have, it won't be updated to 4.4 (see: https://support.google.com/nexus/answer/3468085?hl=en)
But as Zlatty said, you can always install a custom ROM. The plus side to this is the development community hasn't given up on it and have been getting updates out frequently. Plus, you lose the carrier bloat if yours is a Verizon or Sprint GNex. One word of advice to keep in mind if you go this route, though: installing a custom ROM will require a factory reset and has a whole process to go through. You'll lose any data you don't back up.