Drag the margin around the yellow button and move.

Click the yellow button and the icons menu is shown, drag an icon from desktop or explorer, and drop it to a StartEx category.
Or right click a category, and select "Add Icon ..." in the popup menu.

Right click the icon, and select "Delete"

Two ways:

Right click the yellow button, select "New category..."

or Right click the StartEx menu, select "New category..."

Drag the category and drop it to the target place.

Right click the category, select "Rename..." or "Background color..."

Right click the category, select "Delete Category"