To edit an existing image, click the image [1], then click the Image icon [2]. The insert/embed window will appear to make changes.
When you edit an existing image, the URL tab will display the location of the Canvas image; however, the Canvas tab will still display the file. The attributes will remain the same across all tabs and can be edited as necessary.
If you want to replace your Canvas image, click the Canvas tab [3] and locate another image.