Could you describe the problem a bit further? Are you generally unable to edit life dates or just in regard to a specific person? Have you founded the family tree or have you been invited?
Generally, to add dates for birth and death, you need to click on the person in question and then the "Edit [first name's] details" on the sidebar. This allows you to edit first and last names and gives you the option to fill in the birth date either exactly or as "Born before", "Born approximately" etc. Beneath the fields for adding the birth date is a checked box for "This person is living". If you uncheck it, the person will be displayed as dead on the tree and you can add the date of death beneath.
