Hi Christi, Yes you can achieve this. Firstly open up the collage you want to change from the Library > History tab. Next click Undo (up the top right of the screen) until you get back into Collage mode. You can now swap out the images.
The other way to achieve this (without undoing what you’ve done after the collage is complete) is to drag images onto your completed collage from the photo basket, position them and the right mouse click them to “Send to back”.