I modified a certain model in Z position but now the dying/death animation doesn't work properly and the creature is in the air when dies. The death animation has all frames same so this wont be a big problem, but how can I modify the dying animation where the creature falls to the ground so it falls a bit more?
Okay, assuming that's the only animation you need to touch, the easiest way you're going to achieve what you want is to go to the death animation in GMax/Max, zoom into it (so just the frames for the animation are showing), select the object that you originally elevated the Z of (say, the rootdummy), then goto the last keyframe for that object in the time line for that (death) animation.
Then, click on the "Animate" button in the lower right corner of the UI to turn on Animation mode. Drag the object so that the Z is at the new position. Turn off Animation mode by clicking on the button again.
In situations like this what typically happens is it's not just one animation that's been made to look odd because the change. That's the first thing. The second thing is you may not be happy with the way it looks when you're done. The model may appear to fall at an unrealistically-slow speed to the ground. If you need to hasten its descent, go back to that last animaiton keyframe, left click on it to select it, hold down SHIFT and drag a copy of it further up the timeline (so, to the left) till the object lands on the ground at the time you like. You won't need to turn on Animation mode for that operation.