The Message Information tab (screenshot below) contains information that is relevant to the message that is sent to tax authorities as a cover for the report.
All the mandatory data of the report is taken from the data you entered in the forms for Reporting Entity Details and New Report Details and is non-editable. If you want to create a report for some other period, create a new report for this other period. If you want to edit the reporting entity's details, do this on the Reporting Entity Details page. If the reporting period is not a 12-month period, then enter the length of the period in the Warning field.