Just go in and tell them you are unable to make it on your assigned day. Out of the 4 times, I've done 3 walk-ins with no problem being turned away.

Then again, I am pretty charismatic and charming so things just work out. Smile, ask them how they're doing, don't act nervous. Confidence is key. A few weeks ago I did my bios, walk-in, I was talking to the security guys and girl about the Superbowl. We spoke and had a good laugh for about 5 minutes.

Good luck.