How did Julius Caesar change rome from republic to emperor?

If you mean emperor, Julius Caesar was not an emperor. He was a dictator. If you mean empire, Rome was already an empire before Caesar's time. Caesar added to the territories that Rome held. He was also a key figure in the final collapse of the republic, but he, himself did not cause its fall. The fall of the republic had many causes, among them, greed, personal alliances for the advancement of certain families, and out and out corruption. Caesar came into authority by defeating the army of the senate under Pompey. By accepting the honors and in particular the office of dictator, he paved the way for the one man rule of the principate, which happened about 14 years after his death.