How did the entry of the US change WWI?
You did not specify which war you were asking about, so I have guessed that you are asking about WWI. I have changed your question to reflect this and I hope I made the right choice.
The main impact that the entry of the US had on the war was military. Before the US entered the war, the military situation had been one of stalemate. Neither side was able to gain an upper hand. When a large new force enters such a conflict, it of course has a huge impact. The US entry allowed the Allied Powers to win the war in rather short order as the stalemate was broken.
The US entry also had an impact on the politics of the war and, more importantly, of the peace process after the war. President Wilson was determined to push for his "14 Points" agenda. This put many more ideas (such as self-determination, freedom of the seas, and other such idealistic things) on the agenda at the peace conference.