The Union Tool is accessable in the Shapes Tab in the Vector Tools Panel in the Side Panel. It can be used to create a polygon with the combined area of two or more selected polygons, boxes, or circles in the Shapes Layer.
Before using the Union Tool, there must already be at least two features in the Shapes Layer on which to base the union polygon. Features can be created using the Draw Tool, or added from other vector layers loaded onto the map.
To use the Union Tool, follow these steps:
Select the features on which to base the union polygon in the Shapes Layer.
Click the Union button in the Union Tool:
The created union feature is added to the Shapes Layer and replaces the originally selected features.