How can I change the source photo when doing a group merge in PSE 15?

When I select the two photos the best photo that I want to use as my final automatically goes into the "Source" box.  It will not let me switch it no matter what I try, dragging another photo, trying to move it, deleting it etc....  I need to figure out how to get the photo I want into the source box instead.   I am following PSE’s tutorial and I watched a very good tutorial on Utube. Why is a photo automatically going into the source window and how can I get it to change it to the photo I want in the “source window’ keeping in mind I have tried all of the above?