How to convert a part of string to lowercase in c#

You can make use of TextInfo class that Defines text properties and behaviors, such as casing, that is specific to a writing system.

 string inString = "LUXOR".ToLower();
 TextInfo cultInfo = new CultureInfo("en-US", false).TextInfo;
 string output = cultInfo.ToTitleCase(inString);

This snippet will give you Luxor in the variable output. this can also be used to capitalize Each Words First Letter

Another option is using .SubString, for this particular scenario of having a single word input:

string inString = "LUXOR"
string outString = inString.Substring(0, 1).ToUpper() + inString.Substring(1).ToLower();