Under their Parents, press What are you trying to fix? and choose the option saying one of their parents is the wrong person. Mix-ups happen in the best-kept trees! Then say who it should be: someone already on the tree, someone new, or keep a space. For example, if the tree shows Pat as your dad's mum but it should be Rose, choose Rose, and Pat stays safely on the tree for wherever she really belongs.
Where to find it
Person page (Safe edit on) → under Parents, Partners or Children → What are you trying to fix?
Only people with the "Add people" and "Remove" ticks, with Safe edit on.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com