Please give me a bit more information to help you. What is your great grandmother's PID (ID number) in FamilySearch? You should be able to enter the correct person and detach the incorrect person. Did you enter the misinformation through a merge of records of was it just a misentry on your part? Both of these would be handled differently.
Looking forward to hearing from you,
@Greg Raines - If you entered the wrong person as your great grandfather - just edit the information to make him represent the correct person. You usually shouldn't do this but in this case since you created the initial person there is no reason why you shouldn't edit it.
BUT if my entered you really mean you connected the wrong person who was already in tree, then you would want to unlink the individual as the husband in the family. We can help you with that if this is the problem.