Few months back I was given an opportunity by God to be part of Level 1 Course in which I learned the Hong-Sau technique. Intially I was really very thrilled and even I practiced it for 30-40 mins everyday. However few days later I observed that whenever I tried to practice Hong-Sau my mind instead of repeating Hong Sau sounds it started doing "waheguru" simran which is the name given by our Sikh gurus. please help me to know what is right for me to follow and practice.
It may be that your mind is simply reverting by habit to the simran given by the Sikh gurus. The Hong Sau mantra is, by virtue of being a bija mantra, a powerful and important part of the meditation technique that you were taught. However, the technique of watching the breath using a word formula or mantra is to be found in every spiritual tradition in the world. The word formula to be used is not fixed, therefore.
A disciple of Paramhansa Yogananda would naturally wish to use the mantra which he gave. But the Hong sau technique may be used by any person and one need not be a disciple of Paramhansa Yogananda.
As I do not know what your spiritual path and orientation is, I can at least say, therefore, that if you feel it is better and right for you to mentally chant "waheguru" instead of Hong Sau but otherwise practice the meditation technique as you were taught, this is perfectly fine.
On the other hand, if you feel a particular draw to Yogananda or the Ananda teachings, you may also wish to gently persist in using the "Hong Sau" mantra and see if, with a little more practice and concentration, whether this present dilemma won’t resolve itself.