How can I upgrade MacBook Pro 2015 to macOS Serria?

macOS 10.12/Sierra, like OS X 10.9-/10.10/10.11 before it, is available free for download through the App Store application on your MacBook Pro. The default location for the App Store application is in your Applications folder. In order to download through the App Store, you must have an Apple ID. If you already have an iCloud ID, you can use that as your Apple ID as well.

The 10.12 download is pretty big - about 5GB. If your home Internet connection isn't very fast, you might want to go to a cafe, a library/public building, or an Apple Store to get a faster connection for the download. My local Apple Store averages about 50 Mb/second; the last time I downloaded an operating system installer there, it took me about 30 minutes.