How do I upgrade iPhone 7 from iOS 15 to iOS 18?

You do not update an iPhone 7 to iOS 18. For that phone, which originally ran iOS 10, iOS 15 is the end of the line.

If you need or want iOS 18, you will need to purchase a newer phone, preferably an iPhone 15-series or 16-series phone that is likely to have years of updates ahead of it.

Buy iPhone - Apple

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

Apple no longer offers trade-in credit for the iPhone 7, but they probably woulld recycle an iPhone 7 for free.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Apple Trade In - Apple

You can't, sorry.

See why in this compatibility chart:

vaishu213 wrote:

iOS 15.1 update

If you have an iPhone 6s, 6s Plus, 7, 7 Plus, or SE (1st generation), you can update to the latest release of iOS 15. That is currently iOS 15.8.3. You cannot choose iOS 15.1 specifically.

If you have an earlier phone, like an iPhone 5s, 6, or 6 Plus, you cannot update to iOS 15. For those three, iOS 12 is the end of the line, and the ones released before them cannot even update that far.

You will need at least an iPhone 8, 8 Plus, or X to update beyond iOS 15.

iOS 15.1 update