To change the fiscal year in QuickBooks Desktop, begin by opening your company file. Navigate to the "Company" menu and select "Company Information." Next, choose the "Report Information" tab and locate the "Fiscal Year" section. Click on "Change Fiscal Year" and follow the prompts to select your new fiscal year start date. Confirm the changes, and QuickBooks will adjust your reports and financial data accordingly. Remember to back up your company file before making any changes.