How to update your iPhone iOS

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Perhaps to check the Settings in iOS, where updates might be supplied

where and when available, if the qualifiers are met the iPhone may look

to see if a newer system could be available for download & install.

The iOS settings and conditions may not provide a newer or newest iOS

when the older system might be below modern standards for updates.

And that may be your iPhone has older tech, not able to see or use newer

contents; but might still do OK, within its limits. My old iPhone SE(1) does

well enough with iOS 15.8 and newer battery helped immensely.

So when the battery is suspect, by all means see an authorized service

provider for authentic Apple battery replacement. And responsible care

should help it last awhile longer. My newer battery still is 94% after 2 yrs.