I am afraid I am not sure what you mean - are you trying to replace the selected text, to change its style or both?
Generally, to replace the current selection is fairly simple - you just have to call one of the Insert~() methods of RadRichTextBox.
When it comes to formatting part of a document, I suggest you review this article
which explains the concept of Styles in RadRichTextBox. If you are just looking for a way to change one of the properties of the selected text, for example it's ForeColor, you can simply call the respective method of RadRichTextBox:
I hope this is helpful! Let us know if you have additional comments or questions.
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