To change a fiscal year you will follow the steps in the End of Year Check List (from Ledger & Payables, click Reports | End of Year Checklist) during the last month of a short "interim" year. i.e. changing to a new year spanning April-March you would need to be in March of the short year.
When you get to the Archiving step you will first need to change your year. Be sure you have a current Backup of your system before you begin this process.
Go to Options | Define Year and Month...
Click the Change Fiscal Year button.
Read the information and when you have verified that your new year will be correct click the OK button.