Replace changes all instances - that doesn't help you at all.
You could use a Regex:
.NET Regular Expressions | Microsoft Docs[
^] but that could take some time as they aren;t the simplest thing to learn if you haven't met them before, though
Expresso[
^] may help - it's free, and it examines and generates Regular expressions.
The simplest solution is to use the string.IndexOf overloads to get you the position of the two replaceable characters in your string, then use
String.Substring[
^] to break the string around them and concatenation to form a new string with the replaces characters.