how can I get the latest software on my Mac book Sierra 2015

See the following (for MacBook, not MacBook Air or MacBook Pro) from Identify your MacBook model - Apple Support

MacBook (Retina, 12-inch, Early 2015)

Colors: Space gray, gold, silver

Model Identifier: MacBook8,1

Part Numbers: MF855xx/A, MF865xx/A, MJY32xx/A, MJY42xx/A, MK4M2xx/A, MK4N2xx/A

Newest compatible operating system: macOS Big Sur

You can use the following instructions to upgrade: How to download macOS - Apple Support

Note that if you upgrade from Sierra to Catalina or Big Sur, you will also need to update any non-Apple 232-bit apps if you have any. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html