Add photos to Memories in Vs. 15.2.1

Memories are created for me and in edit I can change music and tone of pictures. Also, I can delete photos. But I can't add them. I go to Manage Photos and I can select All or 1. But I want to add other photos from that day and remove some that are in there. It seems as those I can only work with the same 17 photos that were created for me in the Memory.

How do I add more photos to that day in Memories?