Steps:
1. Downloaded cm-10.1-20121229-NIGHTLY-i9100.zip from here and copied to internal SD. Verified MD5 checksum (334a50082024ac57f483fe3ee84fae00).
2. Downloaded gapps-jb-20121212-signed.zip from here and copied to internal SD. Verified MD5 checksum (c4a07a24e11448824b84e314600cd357).
3. Reboot into CWM Recovery.
4. Created backup.
5. Flashed cm-10.1-20121229-NIGHTLY-i9100.zip
6. Flashed gapps-jb-20121212-signed.zip
7. wipe data/factory reset.