I have the latest version of iTunes but my phone won't update

No, perhaps your PC is still running Windows 7, and that version cannot support more recent iTunes versions.

On my Windows 10 PC, the current version of iTunes is 12.12.3.5.

That said, since your SE is still running a very old iOS version (so that you need a computer to update your device), I suggest that you "borrow" a more recent computer from friends/relatives/neighbors or ask for help at an Apple Store or Authorized Service Provider.

Regards

Giulio

On what computer? What happens when you try? Are you getting an error message?