difficulty in shifting to lower gear on shimano (6 speed rear gear) I am new to using a bicycle with multiple speeds. I got this new bicycle 4 days back with Shimano 6 speed rear gears. I don't know much else about the components.

There is no issue when I move to higher gear (2 to 3 , 3 to 4 and so on.)

But I face issue when I try to shift gears from 5 to 4 or from 4 to 3 etc. The chain does not shift and it makes a sound. 

As a trick to get it to shift into the desired gear, I shift to next higher gear when the chain makes sound and then get back to desired gear. e.g. if I am riding in 5th gear and I want to shift to 4th I shift to 3rd for a moment and get back to 4th and then all's well with no more sound from chain. But I am afraid this trick may not be good and that I may be doing something wrong.

So I request guys to suggest me proper way to shift to lower gears.

I tried to find videos on youtube but they didn't help much. The videos show shifting should not be done while pedaling under pressure, so I followed this but shifting was still not smooth.
There is a problem with the cable that links the shifter to the mechanism (rear derailleur)

The cable has become slack over time and needs to be tighter. You can tighten it by turning a barrel adjuster if there is one present. it might be as pictured and there might be one where the cable leaves the shifter on the handlebar. looking in the direction of the arrow, turn the barrel adjuster anti-clockwise to make the cable tighter.

Make small adjustments, ie one quarter turn at a time. You could search for videos to help. It's straightforward when you know how, but difficult and frustrating the first time. Your local bike shop would be happy to help you too.