You can edit any shape after creating it. To do so, click on the shape to open its properties panel. Click on Edit Shape to re-enter the Sketcher mode and sketch the changes.

While creating a shape, if at any time you want to save all of it and go back to the design, click on the Apply option on the left.

If at any time you want to discard all of it and go back to the design, click on the Discard option on the left.