So, if you haven't upgraded to 8.2.0-RELEASE you're fine, just do a normal upgrade to 8.2.0-RELEASE-p1.
If you have upgraded to 8.2.0-RELEASE then you've probably noticed a very annoying bug, it doesn't import ZFS pools at boot time.
What you'll want to do is detach your ZFS volume from the GUI. There are two checkboxes, one for delete shares, one for mark disks as new. Make sure these are both unchecked.
After the volume is detached run the auto-importer and reimport your pool. From there go to system -> settings -> advanced and do a firmware upgrade to 8.2.0-RELEASE-p1. This will fail due to the version check, open the webshell and type in the following commands:
# cd /var/tmp/firmware
# cat firmware.img | sh /root/update && touch /data/need-update && reboot
When the system comes back up confirm it is running FreeNAS 8.2.0-RELEASE-p1