To change the parents for a person on your tree, first make sure that both parents have been added to the tree. Then click the 'edit info' button on the child's node within the Tree. Then click the 'Relationships' tab. Lastly, select the appropriate set of parents from the Parents drop down menu.
If someone needs to be moved to a different branch altogether, you may click the 'more' link within the node and select the 'Move this person' button. You will then notice the node in your 'Move These People' sidebar on the right-hand side of your tree. Navigate your tree to the area you wish to add them to and drag the node to the place you wish to draw the connection, drop them on, select the proper relationship, confirm and you are all set.
You may also use these steps to break a branch and relocate people in a proper generation.