That's something that a good speech therapist can help with. The way to move past a block is to make the sound you don't want to make, because that's the sound you feel like you're going to stutter on and you don't want to do it. To start working on it you have to try in the easiest, safest, least scary situation where you still might block. This can be talking to a pet, to a best friend, making a phone call you don't care about. And the only goal is making some sound in place of where you are blocking.