My husband has a British passport - and a permanent greencard for America. He needs to renew his passport that expires May 2020. Where do we go or what do we to have this renewed. We travel to Scotland every year to visit his family and don't want to run into issues as I have heard you wouldn't be able to travel with that passport if there is less than 6 months to the expiration.
To renew your UK passport, you should follow the instructions on this website. You are likely to be able to renew your passport online, meaning you submit your passport photo online, and post your old passport to the UK passport agency. Alternatively, you can renew your passport when you come back to the UK, providing you'll be in the UK for over three weeks.
There is no minimum validity required on your passport to visit the UK as a British citizen. Indeed, British citizens can be admitted without a valid passport, although it will be difficult to board a flight in this case.
Your husband's residency status in the US is not relevant to his British passport renewal.