I am 5 months pregnant and doctor told me after ultrasound that it's a baby girl.is it possible that the gender would be opposite?what can I do in this regard if I want a baby boy?

It's possible that it is a boy. I have delivered boys to ladies who were told that they were going to have a girl, but it's not common. If you want a boy but it is a girl there is no way to change the gender.