I can’t update my phone using wifi

Normally, update an iPhone will require a Stable, Reliable and Good Wifi connection AND connected to the Charger all at the same time.

On the iPhone, Setting , General, Software Updates and in there one can activate the update in there. If nothing is offered, either the iPhone is already up to date OR the iPhone can not update beyond what it is presently running because the iPhone is too old. Another possible reason - there is not enough Empty Space on the iPhone to allow any available update to download, expand and install.