Could you please try to remove the previous version of the extensions before installing the new one.
Here are the steps:
1. Open Visual Studio and go to Tools -> Extension Manager.
2. Uninstall Telerik WPF VSExtensions and restart the studio.
3. Go again to Tools -> Extension Manager, select the Online Gallery tab and in the search field on the right top corner type Telerik WPF VSExtensions.
4. Download and install them and restart again the studio.
Let me know whether this helped.
the Telerik team