How to install software update on my iPhone 7?

You can update to iOS 15.7.9, you can check by reading this Apple article: iPhone models compatible with iOS 12 -> iOS 15 -> iPhone models compatible with iOS 15.5 – Apple Support (UK)

You need about 8 GB device storage and good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

Go to Settings -> General -> Software Update -> Download and Install.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone or iPad won't update

If your iPhone or iPad won't update - Apple Support 

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

or see: