A New Bedford man has pleaded not guilty to an upgraded charge of murder in the death of his girlfriend’s infant daughter. Ethen Harrison, 23, was arraigned Tuesday in New Bedford District Court. He has been held without bail since pleading not guilty to assault and child endangerment charges. Prosecutor Stephen Nadeau said 9-month-old Ariel Eluziario died from ‘‘blunt force trauma to the head,’’ on Jan. 24, when Harrison was alone with her. The Standard-Times reported that the baby’s mother, Amanda Eluziario, 22, wept in court Tuesday as she sat with the baby’s biological father.