How to import iff files?

Open your Microsoft Excel, and go to the File menu.
Click New, and select Blank workbook.
Click the File menu, and select Open.
Go to the folder location, and look for the file name. You can set the file type to All Files so you can easily find the IIF file.
Click Open.
Go to the File menu, and click Save As.
Choose a location to save the file, and enter the file name. Make sure the Save as type is Excel Workbook.
Click Save.
Then, you can continue importing your CSV file to QuickBooks Online.