Where & How can I download file macOS Sierra ?

Follow the link below,

How to upgrade to macOS Sierra - Apple Support

and if your Mac can run macOS Sierra it can run macOS High Sierra,

How to upgrade to macOS High Sierra - Apple Support

and while you are at it you may want to see if you can upgrade to macOS Mojave,

How to upgrade to macOS Mojave - Apple Support

If upgrading, check to make sure your applications are compatible.        

Application Compatibility

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.