How can I replace one of my saints (top 5 heroes) in the wellspring?

You have to ascend the slacker hero. Make sure to unlock the hero if you have some locked. Found out the hard way. Once you ascend them, the next highest level hero should become Saint.

It's your 5 highest level heroes that determine the 5 wellspring guys. So if you want to swap someone out, you need to manually level the hero you want higher than whoever you're trying to remove. Also, if the hero you want is being leveled by the wellspring, you'll need to remove them first….found this answer on another page.