UptoDate recommends that you cite the UpToDate topic as a chapter in a book titled UpToDate. There are no page numbers to cite, and the publication year for any topic should be the current year.
The example provided by UptoDate:
Marion, DW. Diaphragmatic pacing. In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA. (Accessed on November 25, 2013.)
Unfortunately, UptoDate does not have a direct export to Endnote feature. You will have to enter your citations manually. UptoDate recommends that you cite the UpToDate topic as a chapter or section in a book. In Endnote, choose [References] from the top menu choices. Click on "New Reference" button. Choose Book Section as your Reference Type.