On the Reports tab, select the appropriate object class folder (Items, Requests, Projects/Streams, or Baselines), in the navigation pane. Optionally, expand the folder and select the category of report (Favorite Reports, My Reports, or Public Reports).
a Select a single report in the Reports tab content pane to enable the Modify Report button 
in the Reports tab toolbar.
Then click that button to launch the Report Builder dialog box. This is functionally the same as for a new report except that the various fields will be pre-populated.
b Alternatively, double-click on the report name in the Reports tab content pane to directly launch the Report Builder dialog box.
As an alternative to Step 2 above, select a single report in the Reports tab navigation pane to enable the Modify Report button 
in the Reports tab toolbar.
Then click that button to launch the Report Builder dialog box.
To modify an existing report, modify the various fields in the Edit tab of the Report Builder as described in Creating a Report.
To create a new report based on an existing report, in the Report Name type a new name and then modify the various fields in the Edit tab of the Report Builder dialog box as described in Creating a Report.