Add the paternal grandmother as a mother first, then after she is added as a mother, click on the edit button by their relationship as spouses, from there you can click on remove or replace next to the woman.
You can delete the relationship between them by clicking on the pencil just to the right of their marriage information. Then at the bottom of the pop-up window click Delete Relationship.
Thanks so much to both of you. I was able to correct it.