Steps:
1. Downloaded cm-10.1-20130319-NIGHTLY-i9100.zip and copied to internal SD. Verified MD5 checksum (0feef8bcc50cf87a300bbc566ea2c550).
2. Reboot into CWM Recovery.
3. Created backup.
4. Flashed cm-10.1-20130319-NIGHTLY-i9100.zip
5. wipe cache partition.