How can I unmerge people in Photos? 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).