1. Downloaded cm-10.1-20130601-NIGHTLY-i9100.zip and copied to internal SD. Verified MD5 checksum (cd205d62e52055d7bd9b267244aeb36a).
2. Reboot into CWM Recovery.
3. Created backup.
4. Flashed cm-10.1-20130601-NIGHTLY-i9100.zip
5. wipe cache partition.
6. advanced > Wipe Dalvik Cache.
7. Reboot.
Screen looked weird after flashing (distorted icons and fonts). Not reflected in screenshots. Found this setting under Settings > Advanced > Screen (tab) > Scenario > Video Cold. Set it back to "CyanogenMod (Default)" and everything looks normal again. phew...
