When you open up an account, contact, opportunity, or lead, you'll see the Summary card in the top of the left column.
If you want to change what is displayed here, you can click on the Gear icon and then the Configure Summary menu item. (Note that you must be an administrator to do this)
Then you'll see the list of available attributes on the left and the currently displayed ones on the right.
You can drag-and-drop items from the left column over to the right to change what is displayed.
When you are done, click Save and you will see the Summary Card updated.