Walk on the road from the railway station till you reach Manipal-Udupi highway(5 minutes). Catch any bus going towards left(don't cross the highway) and that bus will drop you at Udupi bus stand in 5 minutes. From Udupi service bus stand, go to the extreme end platform from where buses to Mangalore depart every 5 minutes. Sit in the first or second bus in the queue depending on seat availability. Bus will take 90 minutes to reach Mangalore. Alternatively, you can also walk to KSRTC bus stand which is 2 minutes from private bus stand and can board a Volvo to Mangalore which will cover the distance in 1 hour. Fare is 25Rs. more for Volvo compared to private express buses but is much more comfortable and safe.