If you need to create a new set of parents for an existing individual in the family tree, click on the "Edit Parents" or "Add Parents" button on the individual edit dialog for the existing individual. This opens the Edit Parents dialog. This dialog presents a list of the existing sets of parents for the individual, expressed in terms of the identities of the husband and wife, that is the individual's father and mother. This dialog is a necessary step since an individual may have more than one set of parents. For example an individual may have both biological and adoptive parents.
A frequent situation encountered in research is learning only the names of the mother and father. For example marriage certificates and birth certificates do not generally contain any other information about the parents. However when adding the father and mother as new individuals using this process it is a good idea to put some additional information as a guide to future research, and to distinguish the new individuals from others in your family tree with the same, or very similar, names. For example if the individual to whom you are adding parents was born in the Old World, it is highly likely that the parents were born in the same country, or even in the same geographical district within a country. I also assume that the father was probably no younger than 18 and no older than 60 years old at the time of birth, while for the mother it is extremely unlikely that she was more than 50 years old at the time of birth. Furthermore the parents were probably married within 5 years of the birth of their first child.