My older brother has three boys from the ages 3 years to 4 days... none of them have our last name, if I were to make it possible for my brother to change their last name to ours for a christmas present or something with his permission how EXACTLY would I do this??

Is he on the birth certificate of these kids?

Well first you would have to contact their mother(s) and ask her/them. Is he on the birth certificate? If he isn't on any of them the he isn't even the legal father. He would first have to go to court to have paternaty established. If the children's mother(s) don't agree to it then he will have to go to court and even then the judge won't always order it. Do you by chance have custody of the kids somehow?