My phone is iPhone 7plus

An iPhone 7 Plus can be updated to the most recent iOS 15, and no further. That’s currently iOS 15.8.3.

For iOS 16 and later, you will want a newer (used, refurb) or new iPhone.

To get to 15.8.3: Update iOS on iPhone - Apple Support

If you can’t get to 15.8.3: If your iPhone or iPad won't update - Apple Support

You need an iPhone 8, iPhone 8 Plus or iPhone X if you want to update to iOS 16.

You need an iPhone XR or higher model if you want to update to iOS 18. An iPhone 15 or iPhone 16 would be recommended.