If you have not deleted the return from your iCloud then please go to iCloud.com, log in with your Apple ID, click on iCloud Drive icon, and you will find all files and folders stored on your iCloud. Once you have found your tax return file, just select it and click on the find blue download icon (cloud with arrow heading downwards) on the top, this will typically be the third icon from left.
If you have recently deleted the return from your iCloud then please go to iCloud.com, log in with your Apple ID, click on iCloud Drive icon, and click on the Recently Deleted icon at the bottom right. You will then find all the files deleted within last 30 days, please find your return here and click on recover.