To remove a person you need to break the relationships which connect them to your tree. Remember just because you can see a record does not mean it is connected to your ancestors. There are not unique trees as on other web sites but Family Tree is a common shared tree.
If the person you want to remove is connected as a child then you want to follow the procedure described in the following help center article.
If the person is connected as a spouse you may need to remove children from the incorrect spouse as well as disconnect spouses. The following help center article will help in this situation
What type of "Relationship" are you trying to correct?
Is it a "Couple" Relationship?
Or, is it a "Parent-Child" Relationship?