We have had several good threads over the years, filled with advice and suggestions on "tingly" legs and aching backs. I will link to some here. I would also review all of Taigu's advice on posture in our "Beginner's" series ...
... and this book on posture during Zazen ...
Usually, legs which tingle or "fall asleep" are due putting some pressure on the sciatic nerve ...
My friend, Rev. Nonin Chowaney (Nebraska Zen Center) writes this ...
When my legs begin to "fall asleep", I lightly shift my weight on the Zafu to the left or right (or front or back) as needed to slightly take my weight off the top of my thigh. That seems to work. Also, if sitting in Full or Half Lotus, I will "gassho" and untangle my legs (usually into Burmese) about a minute or two before I need to stand up. The feeling is usually back by that time.
There are many ways to sit zazen: full-lotus, half-lotus, quarter-lotus (with foot on calf), burmese (with both feet on the floor), seiza (Japanese kneeling posture) with the zafu on it's side, seiza on two zafus (one on top of the others), seiza on a bench, and sitting in a chair (this is frequently necessary for those who have injured themselves or with joint replacements). Also, some people with severe physical problems or illness sit zazen lying down.
I recommend to all beginners that they sit as close to full lotus posture as they can for as long as they can. I also suggest that they sit somewhere between wimp and macho. Sit until it becomes uncomfortable, and then sit a few minutes more before you change postures. If you change too soon, you won't stretch out. On the other hand, don't tough it out for so long that you do yourself damage.
Also, learn the difference between soft tissue or muscle pain and nerve pain. Everyone's legs fall asleep from time to time. Sometimes bending forward will take the pressure off the sciatic nerve and the legs will wake up. If your legs are asleep at the end of a sitting and they come back quickly as you stretch them out and get up, I wouldn't worry about it. If they don't and the numbness persists for some time, don't sit the way you have been. You can damage nerves. If you damage 1/8" of a nerve, it can take months to heal.
Anytime you hold the body in a specific position, it will hurt. Just try holding your arm out parallel to the floor for any length of time. Sitting zazen for any length of time will hurt most people, although some can without pain. I have never been very limber, and I sat seiza for three years when I first started while I did exercises and stretched out. Then, I was able to sit burmese style. Eventually I was able to sit quarter-lotus and then half-lotus. I've never been able to sit full-lotus, and as I've aged, I've gone back to quarter lotus. Also, I have a knee problem, and when it flared up severely a couple of years ago, I spent six months sitting in a chair.
Also, is you underwear too tight, your pants pinching your upper thigh or circulation? (This is one reason that we wear loose fitting trousers in Zazen ... and avoid Jeans and such).
Here are some other helpful threads on the subject ...
I might just add that our Zazen is often practice in microcosm for experiencing our whole life. Our lives are sometimes pain, including physical pain. Zazen recognizes that fact. We have to embrace that fact.
So, nothing wrong with trying to make the pain or other unpleasantness go away. We move our legs, shift our posture, do whatever we can. Still, we accept it is there, whether it goes away or not (acceptance without acceptance). No running from the pain, even as we try to walk away from it. If it really will not go away no matter what we do, that is just our life. We just sit with it.
We also learn that, in all cases, there is a great degree of "mind over matter" to pain. Our minds magnify the pain, focus on it. Our minds can also do the opposite. It may not make the pain go away, but mind and body are one.
Remember, pain is not suffering without more ...
All that being said, we also do not do Zazen to the point that there is a real risk of damage to the body. If you overdo with the pain, nerve damage and the like is possible. Even if you need to stand up in the middle of Zazen and do Zazen that way, as walking meditation, no problem. But, to be with a reasonable bit of pain now and then is part of Practice.