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You can try to edit the Template of the RadNumericUpDown control and find the relevant TextBlock and change it with a TextBox. You will also have to synchronize the input text and the value of the NumericUpDown. Please, note that this scenario is not supported out of the box and we have not tested it thoroughly, but at first glance it seems to work fine. I have attached a sample project that demonstrates how you can achieve this.
I hope this helps. Please, let us know if you have further questions.
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