My message says my disk is full on my MacBook Air. One of the largest files is 1.3 GB file from last operating system download. Can I safely delete this file? macOSUpd10.15.1.dmg.download
If that file still has the .download extension, either it is still downloading or ran out of space while downloading.
You could open Finder and click on the Size column to sort. If the folders don't show numbers, click Show View Options under Finder's View menu and check "Calculate all sizes". This may take a bit of time, so be patient if you don't see numbers appear right away. I've noticed in some folders the option was unchecked even though I checked it earlier. Command-J gets you to the option quicker.
If space is being used in hidden files or directories, you can view those with Finder's Shift-Command-period shortcut. It's a toggle.
Folders to check would be your Desktop, Downloads and tmp. If you download lots of Music or Videos you could check those folders as well.
Move what you want to the Trash, and then Empty the trash, otherwise you haven't saved space yet.