If you'd like to customize a current report, we strongly recommend duplicating the report first and then customizing the duplicate.
First, find the report you'd like to customize, then click the "customize" link next to the print options.
Then, in the actions drop down menu, click "Duplicate Report"
After you click Duplicate Report, a screen will prompt you to name the new report. We typically suggest copying the exiting the report's name, and adding "custom" to the beginning, or any other description words to the end for easy identification. Then you can customize it to meet your needs. This will keep the original report intact, but create a new, identical one that you can customize. You can find this new report in the Reports section under the same tab as the original report.
See "How to add a report to an actions drop down menu" if you'd like it to be a printing option for sales or purchases.