The subreport that needs to be modified is named "Contract Payment". Select the date field and in the Properties window click the OutputFormat field to modify. The attached screen shot shows the available formats that display the day before the month in the date string. Once modified, you will just need to bind this subreport to your contract report. This
post shows the elements of the Contract report, and this
post shows how to bind subreports to "parent" reports (step 8). I hope this helps.