Edit RichTextBox programmatically without losing formatting

Question

I have formatted text in the rtf file and I load it to my richTextBox. It works fine, but then I want to remove some text parts programmatically. If I do like that:

richTextBox.LoadFile("TextFile.rtf");
richTextBox.Text = richTextBox1.Text.Substring(fromPosition, length);

text formatting disappears. I've tried to work with richTextBox.Lines, but there is no delete or edit function. All I need is to delete parts of loaded text without losing its formatting. Is it possible?

Thanks in advance.