I would probably use a regex:
public static Regex replaceField2 = new Regex(
"(?<=field2:\").*?(?=\")",
RegexOptions.IgnoreCase
| RegexOptions.CultureInvariant
| RegexOptions.IgnorePatternWhitespace
| RegexOptions.Compiled
);
string result = replaceField2.Replace(InputText,"new value for field2");