Change the speech
Harish congragulatef Vishnu on his success in the examination.
It is a guess:
Vishnu was congratulated by Harish on his success in the examination.
Here I am changing from active voice to passive, I don't know if that is what they asked you to do.
See your later post.
To change the speech, you can rewrite the sentence as follows:
"Harish offered his congratulations to Vishnu for his success in the examination."