Depending on how the child looks, and what symptoms they have, there may be some medical tests to run. Usually psychotherapy is started before any medications are considered. My 5 year old is going through a No phase. Some kids are just particularly vulnerable to being hungry, cold and tired. There are some kids who need enough sleep, or they are very crabby the next day.
Sleep is good.
Time outs are the mainstay of behavioral tips. But there are some very clear rules. For every minute that the time delays going, just start adding one minute and count out loud, you're in your room for 3 minutes. If they don't go, say 4 minutes, if they don't go, say 5 minutes. Set the timer.