You cancel your current extension then apply for a new extension that you meet the requirements for such as retirement or marriage or, failing that, you exit the country and either re-enter Visa Exempt or on Visa on Arrival (whichever one you qualify for) or apply for a Visa from outside of Thailand
All Immigration Offices will do an extension in the last 7 days of a tourism entry stamp but a few including Jomtien and Samui will only do it in the last 7 days.