How do i change the upper angle limit and lower angle limits of a hingejoint2d object?

I get the error: Assets/AngleChanger.cs(35,36): error CS1612: Cannot modify a value type return value of `UnityEngine.HingeJoint2D.limits'. Consider storing the value in a temporary variable

I am trying to change the limits via script.. heres my code:

 public int num;
 public HingeJoint2D angleLimit;
 
 
 void Start () 
 {
     num = 90;
 }



 public void addNum2()
 {
     float num2;
     
     
     if (num > 90 || num == 90) 
     {
         Debug.Log ("You can't go higher than 90 degrees!");
     } 
     else 
     {
         num += 5;
         num2 = (90 - num) * -1;
         angleLimit.limits.max = num2;
         
     }
 }

 public void subtractNum2()
 {
     float num2; 
     
     
     if (num < 0 || num == 0) 
     {
         Debug.Log ("You can't go lower than 0 degrees!");
     } 
     else 
     {
         num -= 5;
         num2 = (90 - num) * -1;
         angleLimit.limits.max = num2;
 
         
     }
     
 }
 

}