character position

i downloded a soldier from asset store but i cant change the position without messing it up
(i was just trying to put his hand on the gun)

Your question is not that clear but,
To move a character, you should move the transform of it’s root or attach a Character Controller to it’s root gameObject and move it.
To only move his hand toward the gun, you should only move the transforms of the bones and should do it in LateUpdate because unity runs animations between Update and LateUpdate and just before rendering so if you do it in Update, then Animations will replace it, i’m not sure it will be done if you don’t have any animations running or not. usually the best way of doing it is to have a base animation and then adjusting the bones in animation based on gun position.

to learn more see character animation videos of unite 10 and it’s example projects. This technique is called IK (inverse kinematics) and is one of the more complex subjects,

Unity Technologies in GDC will introduce it’s new animation system which makes these stuff much easier.