I have the m&p 15 II when i bought it, it did not have a collapseable stock and its pinned but i live in md where they dont have to be. So my question is i bought a replacement stock and a new 6 pin buffer tube do i have to replace the spring in there?

Outside of the stock being pinned, nothing should be different. So you should be able to use the buffer and buffer spring with no problem. Make sure you hold the buffer dent while unscrewing the buffer tube or you will be hunting for pin and/or the spring!