To the left of the Invoice Edit form, you will find 3 tabbed items, all related to the active invoice. These tabs are docked as shown below by default, but you can change the placement and order of these tabs as you see fit. Changing placement to these tabs will be remembered for the next time you start Nexus if you click the Save button in the Layout group. If you aren't liking the new setup you have created, simply click the Reset button in the Layout group to reset placement to the program default.
After rearranging your tabs, click the Save button in the layout group to save your changes for the next session, otherwise the layout will return to the last layout you saved.