You just need to delete the incorrect child/parent relationship or spouse/spouse relationship at the point where they are incorrectly linked to your family lines. Here are 2 links to the Help Centre to show you the steps to do that:
Remember it is the incorrect relationships you are deleteing and not the person. Deleting the incorrect relationship effectively removes the person from the incorrect place in you your family lines without, crucially, deleting them completely from Family Search. Then you can add them back (using their ID) to where they should be in the tree.
If you still have trouble after using the links mentioned above, you can get one-on-one live help (via Zoom) by going to the Los Angeles FamilySearch Library (at www.LAFSL.org), clicking on "Online Consultation" and then scrolling down to "Click here to join now". It will take you to a zoom room with live consultants who can help you. You'll need to share your screen so they can see the problem and guide you through a solution. The library is open 9-5 (CA time), Monday through Saturday.