Hello! If your problem is that the founder's name is simply entered wrongly, click on the person and then "Edit [first name]'s details" and then change the name.
If you want to change the Founder to another person on the tree, click on "Tools" in the menu on the bottom of your screen, and then "Change default person". This gives you the option to change either the person the tree is centered on when loading, the person who is the "Founder", or both.
Hope this helps!
