Steps:
1. Downloaded cm-10.1-20130105-NIGHTLY-i9100.zip and copied to internal SD. Verified MD5 checksum (703318488b8fcccb18e1d2419bdefcdb).
2. Reboot into CWM Recovery.
3. Created backup.
4. Flashed cm-10.1-20130105-NIGHTLY-i9100.zip
5. wipe cache partition.