How to change Symbol Number to Upper Case... in the "richTextBox.Lines" array?

How to change to Upper Case SymbolNumber... in the "richTextBox.Lines" array? I have a problem with "ToUpper()", because it is working only with the "String" data, but I need change the case to upper by number of the symbol.

For example... I have 开发者_JAVA百科a text... "qwerty \n asdfgh \n zxcvbn" ...in the "richTextBox.Text", and I need to change case of symbol #3 in each line (e, d, c) to Upper.

I don't know what's exactly your situation, but you can expand bellow code for words , ...:

I:

var strs = richtextbox.Text.Split("\n".ToCharArray());
var items = "";
foreach(var item in strs)
{
   items += new string(item.Select((x,index)=> index == 2?x.ToUpper():x)
                        .ToArray()) 
                        + "\n";
}

Edit: As I understand from your comment, this would work for you:

indexUpper is input for example set it as 3:

II:

richTextBox1.Lines = richTextBox1.Lines
                .Where(x => x.Length > indexUpper + 1)
                .Select(s => s.Substring(0,indexUpper) 
                            + s[indexUpper].ToString().ToUpper()
                            + s.Substring(indexUpper,s.Length - indexUpper - 1))
                .ToArray();

and if you want to have all items:

III:

             this.richTextBox1.Lines = richTextBox1.Lines
            //.Where(x => x.Length > indexUpper + 1)
            .Select(s => s.Length > indexUpper + 1? s.Substring(0,indexUpper) 
                        + s[indexUpper].ToString().ToUpper()
                        + s.Substring(indexUpper + 1,s.Length - indexUpper - 1)
                        : s.Length == indexUpper + 1?
                        s.Substring(0,indexUpper) 
                        + s[indexUpper].ToString().ToUpper()
                        :s).ToArray();

Try this code:

    public Form1()
    {
        InitializeComponent();
        string text = "qwerty\nasdfgh\nzxcvbn";
        richTextBox1.Text = text;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        //change ever 3rd charater in each line:
        string[] words = richTextBox1.Text.Split(new string[] { "\n" }, StringSplitOptions.RemoveEmptyEntries);
        richTextBox1.Text = "";
        for (int i = 0; i < words.Length; i++)
        {
            char[] chars = words[i].ToCharArray();
            chars[2] = Convert.ToChar(chars[2].ToString().ToUpper());
            string newWord = new string(chars);
            richTextBox1.AppendText(newWord + "\n");
        }
    }