For more information see section 3.15.3 in the AMA Manual of Style, 11th ed.
Use all of the date fields you have information for (published, updated, and accessed). In many cases, access date may be the only available information.
Verify that the link still works as close to submission of your work as possible.
Author(s) (if given). Title of specific item cited (if none given, use the name of the organization responsible for the site). Name of the website. [Date published]. Updated [date]. Accessed [date]. URL