Fusionid,
Just going to the office is not going to do anything for you, neither will phoning in. Heck, I can tell you exactly what you will hear from them.
Have your PI or postdoc write the update letter (preferably the PI), short and sweet: update on what you have been doing in the lab lately and reiterating how they feel about you and why they think you will be a good fit.
I assume you already sent your personal update/letter of intent?
Once you get that done...go to the beach buddy and do not stress over it.
Class does not start for another 5/6 weeks...there is time. Although...I guess I do not know when Davis starts....