My deceased husband's mother and grandparents are incorrect on the family tree. How can this be corrected?
It depends on in what ways they are incorrect.
If information - birth dates, places, etc. - are incorrect simply edit them giving your reasons why they are incorrect.
If the people are completely wrong, you need to detach your husband from them and either connect the correct people (if they are already in Family Tree) or add them as new people.
The following Knowledge Articles may assist
Feel free to post again if you are still unsure what to do.