Editing Shapes

[jsin44]

I'm trying to edit two existing shapes on my project and can't figure out how to do that (first time using the software). On the attached image I want to add a coloured border to the large black circle and adjust the size & dimensions of the white box that the red text is in. I am trying to select the layer and use the rectangle /ellipse tool to start editing but that only makes a new shape as far as I can tell. Any help would be appreciated.

Thanks