1. Click on File > Switch to Single User Mode

2. Create a back up (Maintenance > Backup & Restore > Backup Now)

3. Go to Maintenance > Global Settings > Click on the Financial tab and manually change the "My financial year is from" section accordingly.

4. Save & Close

For example: If your current year end date is Jun 30 and you want to change it to be Dec 31, you will need to set your financial year from Jun 30 to Dec 31 (a 6 month period).  After this 6 months financial year period, it will automatically update to Jan 1 to Dec 31 for regular 12 months financial year for next one.