updating mybb from 1.6.6 to .1.6.8.
Should I first update to 1.6.7 before going to 1.6.8?
No, you can directly upgrade to 1.6.8 without upgrading to 1.6.7 first.
Follow this guide for Up-gradation:
[Wiki: Upgrading] (Broken link, head over to docs.mybb.com instead)