When you change the text of that textblock it would be temporary because it is still bound to the template via a CaptionText property. First thing you need to do is make a copy of the ControlTemplate. Here's how...
- Open your project in Blend
- Right click on the RadUpload control you want to change and select Edit Template
- Select "Edit a copy"
- Expand the visual tree until you see a textblock, select it.
- Change to SplitView and find the Text property (see my attached image)
- Remove the binding and enter the text you want.
Thats it, save your project and run it. From now on, the control will be using your custom template and your text.