Thanks for reaching out to us! The only way to get back into “Collage mode” is to click Undo until you return to it (you’ll see all the collage options appear to the left). This same technique can be used to get back into creating the collage for old edits opened up from the PHOTOS tab (top left) History sub-tab (on the left bar).
Let us know if you have any other questions!
Regards – The Ribbet Team