Replace text in RichTextBox

I am creating a word pad style text editor & I am having problems trying to replace text.

Rather than having a separate form for this I have created a toolstrip with a find textbox (tstxtSearchText), a find button (tsbtnFind), a replace text box (tstxtReplaceText) & a replace button (tsbtnReplace). My find function works fine but I cannot figure out replace.

Here is my find code:
        private void tsbtnFind_Click(object sender, EventArgs e)
        {
            int length = txtMain.TextLength;
            int index = 0;
            int lastIndex = txtMain.Text.LastIndexOf(this.tstxtSearchText.Text);

            while (index < lastIndex)
            {
                txtMain.Find(tstxtSearchText.Text, index, length, RichTextBoxFinds.None);
                txtMain.SelectionBackColor = Color.Yellow;
                index = txtMain.Text.IndexOf(tstxtSearchText.Text, index) + 1;
            }
        }

I have tried the following code for the Replace click event:

 private void tsbuttonReplace_Click(object sender, EventArgs e)
        {
            if (tstxtReplaceText.SelectionLength > 0)
            {
                int start = tstxtReplaceText.SelectionStart;
                int len = tstxtReplaceText.SelectionLength;
                tstxtReplaceText.Text = tstxtSearchText.Text.Remove(start, len);
                txtMain.Text = tstxtReplaceText.Text.Insert(start, tstxtReplaceText.Text);
                txtMain.Focus();
            }
        }

* txtMain is the text editor of the form.

This appears to do nothing. If anyone can see where I am going any help would be greatly appreciated. Thanks in advance.