Steps:
1. Downloaded cm-10.1-20130103-NIGHTLY-i9100.zip and copied to internal SD. Verified MD5 checksum (e3962398f83f6831c58c3f2ecb3aa724).
2. Reboot into CWM Recovery.
3. Created backup.
4. Flashed cm-10.1-20130103-NIGHTLY-i9100.zip
5. wipe cache partition.