change y rotation unity code example

Example 1: how to just aet z rotation on transform unity

transform.rotation = Quaternion.Euler(0, 90, 0);

Example 2: how to change rotate with script unity

var rotationVector = transform.rotation.eulerAngles;
rotationVector.z = 0;  //this number is the degree of rotation around Z Axis
transform.rotation = Quaternion.Euler(rotationVector);
//if you put this in a coroutine and yielding for some amount of time 
//you can have something like a rotating loading icon