Just making a simple button to make my player rotate when you press a button(on the canvas not the keyboard). So I made this function:

public void Rotate()
{   
   transform.rotation = Quaternion.Euler(transform.rotation.x, transform.rotation.y - 90, transform.rotation.z);
}

and connected it to my button.

but every time I press the button, it doesn't turn -90 dergrees but it rotates to a strange rotation like 0, -88.6758, 0 and if I press it again it doesn't turn! pls help!

Instead of transform.rotation.y in the y spot of the euler use transfom.eulerangles.y.

A better way is to keep track of your rotation angle and then in the update method, use 

        Vector3 rotatedVector = Quaternion.AngleAxis(currentAngle, Vector3.up);