I want to update my iPhone

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. 

Hey there,

what is your iPhone model?

and what do you see when you go to Settings > General > software update?

if you have iPhone 6/6plus the last update for this device’s is iOS 12.5.5 you can’t update it beyond!