@Rus****
, IF you're on the original 90 day entry of the ED visa you got you don't need to cancel it. You only need to do it IF you are on an extension of stay from the immigration office.
You're previous post makes me think you're not (seeing as you didn't even know you COULD extend your ED visa ;) )
Most people enroll in schools, get the documentation to go to a thai consulate and get a 90 day, single entry ED visa. Then when that 90 days is running out they get more documentation from their school and go to the immigration office to get a 90 day extension. You do that for the whole year.