What should I do if I need to cancel my expired Non B visa before applying for an extension in Thailand?

As a rule, extensions based on employment, studying, etc have to be cancelled officially for you to be able to apply for a new non-immigrant-based extension regardless of whether it expires or not.

Otherwise, you cannot apply for an extension for any reason.

Extensions can be cancelled only by the immigration office that issued them. It doesn't matter whether you quit already, the extension has expired, or you travelled out of Thailand, or not.

You get proof or receipt of your old work permit cancellation from the labour office that issued it, then go to that immigration office to apply for cancellation of an extension. There would be possibility you get fined for late cancellation (often but not always), the same rate as overstaying. You may or may not need to get a termination letter from your previous employer depending on when you quit the last job.

You don't have much choice other than following the directions of the current immigration office.

This is why we always advise members to cancel an extension officially at the immigration office before you leave regardless of whether it would expire or not, unless you are not planning to be back in Thailand on a long-term basis.