I am pasting the answer here to make it available for our community in case someone else encounters this question:
The behavior is the same as in the UI. When you select text and you start typing the old text is removed and new text is inserted.
Having this in mind the available approaches to achieve your scenario are two:
1. Delete the text using the RadRichTextBox.Delete method and insert the new text.
2. Or just simply insert the new text using RadRichTextBox.Insert method.
Regards,
Tanya
Telerik
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