Hi Terri, Thanks for contacting us and sorry to hear that you’ve run into this issue! Currently the only way to edit the images in your collage is to open the item from the Library > History tab (as you have), and then to continue to click Undo until the Collage tab appears again up the top center in place of the Edit tab. This unfortunately means undoing any changes that were made after the collage was completed (such as adding text and effects). Once back in this “Collage mode” you will be able to change out the images.
Let us know if you have any further questions or problems!