To find the Object-specific Activity Log, simply go to the Objects tab from the top navigation bar. Once you have clicked into an object, you will see Activity Log listed on the left side menu.
This will bring you to a log of changes associated with this specific object. The changes made will be categorized into sections chronologically by the date of the change, the most recent being at the top. You can find the exact date and time the change was made on the left hand column. The center column lists which user administered the change, and the far right column displays the details of the change made to the object. Filter by field, time, and user to narrow down your search.