To customise a report, simply click on the Customise button for the standard report which is closest to what you are looking for.
For example if you wanted to download a report of all unpaid and incomplete registrations, you will want to customise the Simple Participant report.
There are 2 sections of customise reports, Report Fields and Filtering
Under the Filtering section you can deselect Paid and Re-Entered and instead select Unpaid and Incomplete. Once you are happy with the data the report will be capturing, change the report name and description so you don't forget what the report is and then press save.
The system will then take you back to the Reports home page, where you will now see your customised report and can download the report to your computer.