what jaap did can be done by transform.rotate and i think it's not what you want and you want to set the value directly. you can use his code but remove the "rot*" and then the code will be

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

if you want to add some value to any of the x,y,z values simply use

transform.Rotate (0,0,10);

this will add 10 to Z value

I think this is what you're looking for :

// Sets the transforms rotation to rotate 30 degrees around the y-axis
transform.rotation = Quaternion.AngleAxis(30, Vector3.up);

Don't know what you exactly want but maybe this is it:

var rot = transform.rotation;
transform.rotation = rot * Quaternion.Euler(0, 90, 0); // this is 90 degrees around y axis

The rotations are stored as quaternions, so you can store the appropriate quaternion and set transform.rotation to that.

Perhaps an explanation of what you are trying to accomplish would help. If you go into "Debug mode" and watch the rotation values change, you can see how just changing one axis's value isn't really more efficient. It only looks that way in "Normal mode".