Hi I purchased r/t tickets on Vietnam airlines for manila to Saigon - Saigon to Hanoi and Hanoi the nha trang.

I want to cancel all flight tickets right away. I have purchased all tickets online using a credit card within the past 2 weeks. My flights are for next month. Who can I contact to cancel my flights...
I don’t have a office in my hometown, is there is number I can call to cancel?