My average heart rate even at rest is as high as 90 to 100 bpm, what is the best way to reduce it to about 70?

Why? Your heart is regulated by a system of fine neurohumoral mechanisms - why are you trying to defeat them? If your resting hr is 90-100, there's a reason for it. That said, generally the better shape you're in, the slower your resting hr so working out and being keeping body mass index 18-25 will usually do the trick. Fever, anemia, anxiety, stress, exercise, xs thyroid etc. Increases it.