Enter the report definition page using the icon at the top right-hand side. (Show Report Definition)

If the source on the report does not start with COSxxxxx (which is a programmed report) it is possible to add fields, or edit existing fields, by opening the report to status 1 using the option: ‘Make edit version’. 

Enter the Fields folder and you will be able to add fields using the icon at the bottom left hand-side, edit the format with the toggle icon on the right of each field, or move existing fields up&down using drag&drop or the icon on the right of the field in the grid. 

When finished modifying, you have the option to register using the option: Make Main Version.