How to adjust foot rest? I cannot fold the foot rest.
I believe you mean the leg rest, the adjustable thing for your baby's legs, don't you:
There are TWO little levers on the sides of the leg rest which you need to push and then move the legrest down (or up, but up goes even without pushing). The same thing should also be n your stroller manual which I recommend keeping and consulting.
Here's a video (check about 1:12 where the lady adjusts the leg rest) to help you out, too: