Hi Brijesh,
Thank you for contacting Syncfusion support.
We have checked your query "How to override the default TabItemExt Header Template in TabControlExt". To change the TabItemExt style to Metro look, you can customize the TabItemExt style of TabControlExt. We have prepared sample for the same in which we have customized the Border of TabItemExt Header.
You can download the sample from the below location.
Sample:
TabControlExt_Sample
If we misunderstood your requirement, please provide us screenshot regarding the requirement. This would help us to proceed further.
Regards,
Subashini M.