Why did President Grant change his name?

Grant’s parents named him Hiram but called him by his middle name, Ulysses, or “Lyss” for short. When he entered West Point in 1839, his name was mistakenly changed to U. Simpson. He then became known as U.S. Grant, rather than H.U. Grant, which stood for the initials for his given name, Hiram Ulysses.