Use of Wi-Fi is expected and generally required.
For Wi-Fi: Coffee shop, hotel lobby, library, cafe, Apple Store, most fast food shops, your cellular provider’s store, etc.
Your only options are Wi-Fi, or to use a computer (possibly via your iPhone as a hotspot to stage the update), or to use a hotspot.
Why? The cellular providers do not want large wads of data downloaded via cellular, as they’d have to enlarge their networks to support that. And for many folks, an iPhone update would blow through their data caps. Cellular data costs more. Sometimes much more.
Update (variously very expensive) via hotspot: https://mashtips.com/update-iphone-without-wifi-using-cellular-data/