The angle of dip is the angle between earth's horizontal magnetic field and magnetic field produce by coil .
But I could not understand when current is reversed the angle of dip changes but magnitude of magnetic field is same .
The angle of dip is the angle between the horizontal and the vector sum of the magnetic fields from the Earth and the coil. Reversing the current in the coil reverses the field from the coil and changes the vector sum. You will need to draw vector diagrams.