Easy.   Load your model.  Change from the objects tab to the Links tab (last in the row).   All images used will be listed there.  Find the one you want to edit, right click and "Export Link".  Save the image.   Then you  can edit it to what you want.  

If you change it and want to save your newly modified object, would be wise to save as a different name fromthe original just in case you want to use the old one again.

Gill