Can I update my phone on public WiFi?

AS LONG as the update is available over WiFi (some very rare cases now, but if the carrier blocks the update to be made over mobile data only, you won’t be able to use any WiFi network for that).

Is it safe to use public WiFi to update iPhone?

Don’t use public Wi-Fi hotspots, and if you really have to, make sure you use a reputable VPN. It’s still as simple as that. Sometimes a hotspot might be a malicious network with a generic name, “public free Wi-Fi” or similar.

Is it safe to download on public WiFi?

Public Wi-Fi Isn’t Secure

If the network isn’t secure, and you log into an unencrypted site — or a site that uses encryption only on the sign-in page — other users on the network can see what you see and send. They could hijack your session and log in as you.