Check out example codes for "how to change rotate with script unity". It will help you in understanding the concepts better.

Code Example 1

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

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