How to Change an Existing TFS 2010 Report
Sometimes the default reports that come with the TFS process templates are not 100% compatible with the clients needs or desires. Luckily for us these reports can be changed customized as we wish.
1) First of all we must open the process template manager, and download the same process template that the team project in which we want to change the report uses.

2) Do steps 1 to 4 from this post How to Add TFS 2010 Report to a Team Project.
3) After these steps instead of creating a new report, go to the location that you downloaded the process template, go into the reports folder and choose the report you wish to change.
4) you can make whatever modifications you want to the report and add it to the team project as it described in steps 6 to 8 of this post How to Add TFS 2010 Report to a Team Project.
5) Change the data source of the report in the report site to the shared data source. Do this by entering the report you changed in the site, go to the “Properties” tab and click ”Data Sources” in the left side menu, then click the “Browse” button. choose a shared data source and click on the “OK” button.
IMPORTANT!! You must click the “Apply” button! it not your changes will not go into effect.

Enjoy the modified report .