From the Creatives page, mouseover the thumbnail of the static creative you want to edit and click on the Edit button, as shown below.

By clicking on the Edit all formats radio button, you can toggle if you want your edits to apply to the selected format or all the formats.

By opening the layers panel, you can select individual layers of the ad and depending on the layer type, you will see a different option bar. By clicking and dragging on the 6 dot icon for each layer, you can reorder the layers and by clicking on the 3 dot icon, you can see all of the available layer actions, as shown below.

You can also select items by clicking on them in the stage. You can then drag them about using the mouse, or nudge them using the arrow keys. By pressing the backspace you can delete the item, but don't worry, you can always click on the Undo button, as shown below.

By selecting a non text element, you can click on the option to add an effect. The video below shows adding a drop shadow to a button.

The video below shows you how to replace a graphic by selecting the graphic and clicking on the Replace button in the options bar. Then, you are able to select a new graphic to use. Once again, you can always use the undo button if you prefer it as it was.

To add new content to your ad, you can select one of these 4 options.

The depending which of the 4 options you choose the panel to the right will change to give you further options of what to add to the layer

Great, you are ready to go and edit a static creative for yourself!