The information is at: http://www.housing.ucdavis.edu/dining/meal_plan_changes.asp
There is a meal plan change period each quarter, and you can only change your dining plan during that period.
However, there's a calendar at: http://housing.ucdavis.edu/calendar/
It says that Saturday, December 1 is the first day to change meal plans for the winter quarter, and Friday, January 4 is the last day. So you should be able to do it.
There are instructions at: http://housing.ucdavis.edu/dining/meal_plan_changes.asp
It says that to change your plan using MyUCDavis:
1.Log into MyUCDavis
2.Select the "View My Information" link under the My Student Housing channel to open a window titled "Your Housing Contract Information"
3.Select the "[Change Meal Plan] link under Meal Plan to view a Meal Plan Summary
4.From the menu at the left, select "Change Mealplan" to display the existing (current) plan and a pull-down menu titled "New Plan"
5.Select a different plan from the "New Plan" pull-down menu; make note of the cost of proposed change
6.Select "Submit Plan Change" to approve the meal plan change
7.To Cancel the meal plan change, select "Cancel"
It also says you can make the change at any Area Service Desk, so if I were you and having trouble figuring out how to do it at MyUCDavis, that's where I'd go next.
I'm sorry; I'm not connected to UC Davis in any way, so I don't have any direct experience with how to do it.
Good luck getting it sorted out.