C# Replace HEX in binary file

I have a binary file, where are a few values what should be changed. To be more exact, at two part of the file, in the be开发者_运维知识库ginning, there are two HEX values

66 73 69 6D 35 2E 36 39

what should be changed to

4D 53 57 49 4E 34 2E 31

How could I do this async, and as fast as possible? I've got to the point where I read the whole file into a byte[] array, but this class has no search or replace feature.

Here is a method I wrote which you can use to find where in your byte[] the bytes are that you are trying to find.

/// <summary>
/// Searches the current array for a specified subarray and returns the index
/// of the first occurrence, or -1 if not found.
/// </summary>
/// <param name="sourceArray">Array in which to search for the
/// subarray.</param>
/// <param name="findWhat">Subarray to search for.</param>
/// <param name="startIndex">Index in <paramref name="sourceArray"/> at which
/// to start searching.</param>
/// <param name="sourceLength">Maximum length of the source array to search.
/// The greatest index that can be returned is this minus the length of
/// <paramref name="findWhat"/>.</param>
public static int IndexOfSubarray<T>(this T[] sourceArray, T[] findWhat,
        int startIndex, int sourceLength) where T : IEquatable<T>
{
    if (sourceArray == null)
        throw new ArgumentNullException("sourceArray");
    if (findWhat == null)
        throw new ArgumentNullException("findWhat");
    if (startIndex < 0 || startIndex > sourceArray.Length)
        throw new ArgumentOutOfRangeException();
    var maxIndex = sourceLength - findWhat.Length;
    for (int i = startIndex; i <= maxIndex; i++)
    {
        if (sourceArray.SubarrayEquals(i, findWhat, 0, findWhat.Length))
            return i;
    }
    return -1;
}

/// <summary>Determines whether the two arrays contain the same content in the
/// specified location.</summary>
public static bool SubarrayEquals<T>(this T[] sourceArray,
        int sourceStartIndex, T[] otherArray, int otherStartIndex, int length)
        where T : IEquatable<T>
{
    if (sourceArray == null)
        throw new ArgumentNullException("sourceArray");
    if (otherArray == null)
        throw new ArgumentNullException("otherArray");
    if (sourceStartIndex < 0 || length < 0 || otherStartIndex < 0 ||
        sourceStartIndex + length > sourceArray.Length ||
        otherStartIndex + length > otherArray.Length)
        throw new ArgumentOutOfRangeException();

    for (int i = 0; i < length; i++)
    {
        if (!sourceArray[sourceStartIndex + i]
            .Equals(otherArray[otherStartIndex + i]))
            return false;
    }
    return true;
}