Can I upgrade an iPhone 5S directly to 8.3 from 7.1.3?
I presume you mean 8.1.3 … &amp; also 7.1.2, as there never was a 7.1.3 afaik

You can go straight to it - in fact, there's no other option. It's the only route.
Apple stops signing old releases shortly after a new one comes out, so an older version wouldn't work, even if you got hold of it.

Safest route to any update is…


Sync to iTunes
Backup to iTunes
Download the update from iTunes, without updating.
Update as a separate step [ensures the download is full &amp; complete before starting]


Belt &amp; braces - you have a full, recent backup of the current state should anything go wrong.