How to replace list item in best way

Question

if (listofelements.Contains(valueFieldValue.ToString()))
{
    listofelements[listofelements.IndexOf(valueFieldValue.ToString())] = value.ToString();
}

Answer 1

Or, building on Rusian L.'s suggestion, if the item you're searching for can be in the list more than once::

[Extension()]
public void ReplaceAll(List input, T search, T replace)
{
    int i = 0;
    do {
        i = input.FindIndex(i, s => EqualityComparer.Default.Equals(s, search));

        if (i > -1) {
            FileSystem.input(i) = replace;
            continue;
        }

        break;  
    } while (true);
}