We recommend that instead of editing a default report or dashboard, you save a copy of it.
To edit a default report:
Click the Reports tab.
If you have not already done so, click the new report folder icon and create a new report folder to house your edited report. See the Salesforce Help for more information.
On the reports tab, select the report that you want to edit.
Click Save As
Enter a name and description for your report.
Choose the folder that you created earlier in the Report Folder drop-down list.
Click Save & Return to Report.
Edit the report. See the Salesforce Help for more information.
To edit a default dashboard:
If you have not already done so, create a folder to house your edited dashboard. See the Salesforce Help for more information.
Click the Dashboards tab.
Open the dashboard that you want to edit.
Click Edit.
Click Save As.
Enter a title for your dashboard.
Choose the folder that you created for the dashboard in the Save to drop-down list.
Click Save and Run Dashboard.
Edit the dashboard. See the Salesforce Help for more information.