You can do what you are describing by navigating to the Customize tab, and clicking on the Tabs tab link.
From here click on Edit Tab Links... for the Tab you are attempting to reorder the tab links for.
On the Scren which pops up, you can Change which screen will be opened when clicking on a particular tab link by selecting the dropdown beside that Tab Links Name(underneath the column labelled Opens Screen) . You can also edit the order which these Tab Links will appear by using the up and down arrows on the Right hand side of this Screen. (Located between Move and Delete)