Replace

public static String Replace(this String originalString, String oldValue, String newValue, StringComparison comparisonType)
{
	Int32 startIndex = 0;

	while (true)
	{
		startIndex = originalString.IndexOf(oldValue, startIndex, comparisonType);
				
		if (startIndex < 0)
		{
			break;
		}

		originalString = String.Concat(originalString.Substring(0, startIndex), newValue, originalString.Substring(startIndex + oldValue.Length));

		startIndex += newValue.Length;
	}

	return (originalString);
}

String newString = "AbC".Replace("b", "B", StringComparison.OrdinalIgnoreCase);