Edit master shapes
When you need a specific shape, find a shape that’s close and customize it to suit your needs.
Right-click a shape, select Add to My Shapes, and choose the stencil where you want to store the new shape.
Open the stencil, right-click its title, and select Edit Stencil.
Right-click the shape, select Rename Master, and type a new name for the shape.
Right-click the shape, select Edit Master > Edit Master Shape, and make changes to the shape.
Select Close Window.
Select Yes.
Select Save in the stencil title bar.
Source: Microsoft