Change some value inside the List<T>

You could use ForEach, but you have to convert the IEnumerable<T> to a List<T> first.

list.Where(w => w.Name == "height").ToList().ForEach(s => s.Value = 30);

I'd probably go with this (I know its not pure linq), keep a reference to the original list if you want to retain all items, and you should find the updated values are in there:

 foreach (var mc in list.Where(x => x.Name == "height"))  
     mc.Value = 30;

You could use a projection with a statement lambda, but the original foreach loop is more readable and is editing the list in place rather than creating a new list.

var result = list.Select(i => 
   { 
      if (i.Name == "height") i.Value = 30;
      return i; 
   }).ToList();

Extension Method

public static IEnumerable<MyClass> SetHeights(
    this IEnumerable<MyClass> source, int value)
{
    foreach (var item in source)
    {
       if (item.Name == "height")
       {
           item.Value = value;
       }

       yield return item;
    } 
}

var result = list.SetHeights(30).ToList();