The file should be in
~/Library/iTunes/iPhone Software Updates not
/Library/iTunes/iPhone Software Updates
The first is your personal User Library, the second is the System Library.
The tilde ~ sign is an abbreviation recognised by the System meaning
Boot Drive/Users/Your Name/ & saves having to know the names of the boot drive & current user.
It's kind of the Mac equivalent of using
%appdata% in Windows to find the current user's Roaming folder - an environment variable.
~/Library is hidden, so delving down the file hierarchy manually you won't find it.
You can get to
~/Library from the Finder > Go menu if you hold Opt ⌥ whilst selecting the folder, but if you have a specific destination, I find it easier to Go directly there…
- In Finder, Cmd ⌘ N for New window
- Cmd ⌘ Shift ⇧ G for Go…
~/Library/iTunes/iPhone Software Updates including the tilde ~ & hit Enter ⌅