How to update iOS on my iPhone?

The iPhone 7 Plus cannot be updated past the iOS 15s. The hardware is not capable. If I am misunderstanding and you are asking how to update to iOS 15, then here are the instructions: Select Settings > General > Software Update. If your iPhone is eligible for an update, you will be prompted to do so.

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.