I was dating two guys when I got pregnant and I already have two kids by one of the guys but my third child does not look like my other kids father he look like the other guy. I let the father of my others kids sign the birth certifate but now I'm with the other guy that my son looks just like and he has been there for him since day one. The guy that my son has the last mnae does nothing for him and my son only knows the guy I'm with as daddy. How do I get his last name chaneged from the other guys??

First thing that is going to need to happen is that you are going to have to sort out paternity. That means DNA testing.

Depends on where you live, the guy that signed the B/C might be the legal father. If he is not also the bio father, that needs to get sorted out.

AFTER all of that, you get the legal father to go with you to the courthouse and file the paperwork. It's a simple thing if both parents consent.