I want to update my iPhone

You can update your iPhone to iOS 15, see this Apple article:

iPhone models compatible with iOS 15.2 – Apple Support (UK)

Go to Settings -> General -> Software update.

You need about 7 GB device storage.

You can do the update via PC, See this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

 If they are On, turn Off Hotspot & VPN.