I would use the custom report in FTM to find all the people with "living"
Then you can go about editing them
I find the best way to categorise living people is to leave the date of Death field open. FTM will interpret this as a living person unless the person is over 113 year old
What is even more important is to "kill off" dead ancestors in FTM. If you don't have a Dod but know the person is deceased then type in "?" into the DOD field and FTM will convert this to "Unknown"
I have found that the new year range option in the index in FTM a wonderful way of quickly identifying those deceased people that need to have their records adjusted.
Also, when you are marking one or both person in a marriage as deceased don't forget to also update the marriage status from "Ongoing" to "Death of one spouse".
Run a marriage report to see who have died but FTM is still showing their marriage as "ongoing" which is obviously absurd.