I need to update my phone

Start at: Update your iPhone, iPad, or iPod touch - Apple Support

If you're on an iPhone 6, iPhone 6 Plus, or earlier, you'll be able to use iOS 12 (currently 12.5.4), but no newer versions.

There are no hardware upgrades that will allow older iPhones to use newer iOS.

If on an iPhone 6, iPhone 6 Plus, or earlier, your options for iOS 13, iOS 14, and the just-released iOS 15 are a newer (used, refurb) iPhone, or a new iPhone.

If you are on an iPhone 6s, follow the instructions from the hyperlink in a previous reply for instructions. If you're short on storage on your iPhone 6s or newer, you might need to use a computer to perform the update. How to do that is included in the linked article.