If you are generating accounts within the Accounts Production software, it will automatically apply the wording on the cover page as to whether it is a year or period end.
The Companies Act 2006 states "A company's financial year ends with the last day of that period or such other date, not more than seven days before or after the end of that period, as the directors may determine." This means that if your accounts are within 358-372 days (or 359-373 for a leap year), company law states they should state year instead of period. If it is any more or less than the number of days indicated above then the accounts will be required to state period.
To adjust this manually, you can open the period in Accounts Production click on Accounts Presentation > Financial Statements > Accounting Period Reference
For more information on the legislation, click on the link for The Companies Act 2006 Section 390.