In the report menu check the box to show the mass changes control
This will show a new section below the report header:
Choose the records to modify by checking the box in the leftmost column:
You can also select all the visible rows using the checkbox in the leftmost column of the header row.
The pager control at the bottom allows you to select more rows to show on the page at once.
Chose the field you want to modify on the selected records:
You can select either from the visible fields shown in the report, or you can modify a field not included in the report.
Enter the value
The value to change to will depend on the type of the field you select:
Modify the records
Finally click on the Modify Records button and confirm the change
The confirmation dialog will indicate how many rows will be affected by the change