First of all, you need a ClockworkMod recovery that you can install the usual way, but also through the ROM manager. The procedure will delete personal information, but not the contents of internal memory.
- Download Jelly Bean ROM
- Move it to the phone's internal memory.
- Turn off your phone.
- Start the phone in recovery mode.
- Once the menu appears, enter your organization and recovery "install zip from SD card"
- Select Wipe & factory reset.
- Flash the ROM.
- Select Reboot System.
And there you have it ... Jelly Bean on Galaxy Nexus