Am not able to update my iphone

Your iPhone 7 Plus can update as far as iOS 15.7.3

Here are two different ways to ensure the highest iOS that your device will support is installed on your device.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Make sure you are following the directions carefully.

If your Apple mobile device won't update, read this support article --> If your iPhone or iPad won't update - Apple Support