How to replace text in string in C#?

Short answer (guessing a bit):

text = text.Replace("\xFFFD", "?");

And learn about Unicode and character encodings, especially utf-8.

Long answer:

Well, do you mean "\xEF\xBF\xBD" literally? That is, a string consisting of these characters:

backslash, uppercase latin character E, uppercase latin character F, backslash, uppercase latin character B, uppercase latin character F, backslash, uppercase latin character B, uppercase latin character D

Then, the answer would be:

text = text.Replace(@"\xEF\xBF\xBD", "?");

Or do you sequences of characters which are described by the C# escape sequence "\xEF\xBF\xBD", namely:

LATIN SMALL LETTER I WITH DIAERESIS, INVERTED QUESTION MARK, VULGAR FRACTION ONE HALF

(which would be displayed as "�)? Then, the your code would be correct:

text = text.Replace("\xEF\xBF\xBD", "?");

Or do you want to replace the byte sequence

EF BF BD

(which could actually be the utf-8 representation of the unicode replacement character, FFFD, which is often displayed as"�")?

This is just a wild guess, but by intuition says you actually want to achieve the latter. Now, a .Net string contains characters, not bytes, but assuming that you have read these bytes e.g. from a file as utf-8, the answer would be:

text = text.Replace("\xFFFD", "?");