I wanted to make a mixed video of all the people in my pictures but accidentally merged them all as one.
What can I do to un-merge them as seperate people again?
Tap on the existing (mixed) person. Tap Select at the upper-right, then tap on each photo that is not the person whose name is at the top of the screen. Tap on the Share icon at the lower-left, then tap "Not this person" on the bottom row of options. You may need to scroll to see it.
This will remove them from the current person. Where they go from there is a separate issue. I haven't found a way to manually tag a photo as being a particular person, but it's likely they should reappear in the Add People section. You may need to do further work to get all photos of a particular person back together at that point (i.e. merging and using "not this person" as needed).
After following the other answer, the problem that I had was the person I removed from the mistakenly merged profile could no longer be found under the people profile section so I was unable to create a new profile for him.
I called Apple help desk and they asked me to simply take another photo of the person with my phone or add another image of the person to the phone. The facial recognition software then detected the new photo and created a new people profile for this person again. Worked like a charm!