public void ReplaceCharInString(ref String str, int index, Char newSymb)
{
    str = str.Remove(index, 1).Insert(index, newSymb.ToString());
}
 
// либо:
public String ReplaceCharInString(String str, int index, Char newSymb)
{
    return str.Remove(index, 1).Insert(index, newSymb.ToString());
}

public string ReplaceCharInString(String source, int index, Char newSymb)
{
    return String.Concat(source.Substring(0, index), newSymb, source.Substring(index + 1));
}

public String ReplaceCharInString(String source, int index, Char newSymb)
{
    char[] chars = source.ToCharArray();
    chars[index] = newSymb;
    return new String(chars);
}

public String ReplaceCharInString4(String source, int index, Char newSymb)
{
    StringBuilder sb = new StringBuilder(source);
    sb[index] = newSymb;
    return sb.ToString();
}

public void ReplaceCharInString(String source, string oldSymb, string newSymb)
{
    return System.Text.RegularExpressions.Regex.Replace(source, oldSymb, newSymb);
}

public unsafe void ReplaceCharInString(ref String source, int index, Char newSymb)
{
    if (index < 0 || index >= source.Length)
        throw new IndexOutOfRangeException();
    fixed (char* ptr = source)
        ptr[index] = newSymb;
}