Open IMDB.com on a computer, log in with your IMDB account, open your name page and then click the Edit page button at the bottom of the listing. From the list of available fields to correct select "Name correction or merge" at the top, and choose "Add correction" in the drop down box next to it, and click the Continue button at the bottom of the page. Follow the instructions to correct your name the way it should be.
I'm not sure it will go through, though. If you are actually credited as P.J. Phonangdara in the movie credits then your credit is considered correct, especially as it is your only credit. If your name change does not go through you can add your Birth Name under Biographical Information instead, from the same list of available fields to edit as I mentioned above.