SALEM – A Lynn man about to go on trial for beating his girlfriend in front of her two children changed his plea to guilty.He was sentenced to two to six years in state prison.Michael Gallagher, 55, of 150 Marianna St., pleaded guilty to two counts of assault and battery with a dangerous weapon and three counts of assault and battery before Salem Superior Court Judge Howard J. Whitehead.The charges concern an incident at his home in March when he beat his 38-year-old girlfriend in front of her two children, ages 5 and 11, following a night out together.The victim, a resident of Stoneham, told authorities she was first assaulted in the car, then on the stairs and finally in the living room with a telephone and entertainment center.The two had a relationship for about two years before the incident.Assistant District Attorney Melissa Woodard argued for a state prison term of four to six years, based on the defendant’s record and the victim’s two minor children witnessing the incident.Defense lawyer John V. Apruzzese proposed two years in prison, pointing out that the victim sustained minor injuries, that she was a reluctant witness and has remained in contact with his client since the event.