Here is a scenario. Navigate to Solutions->Entity->Form . Here we are considering an Account form.
By default the "Body" Button on the ribbon would be enabled. But we would like to edit left navigation items. So click on the Navigation button which is on ribbon as shown below.
Please note the left navigation items are enabled for edit. "Common" is the first section and "Sales" is the second one.
Reordered left navigation items and section could be seen in the following screen shot. The section "Sales" in the left navigation was dragged and dropped to the top in Left Navigation items.
Its also possible to remove left navigation items by selecting the required one and then click the "Remove" button on the ribbon.