How do I upgrade from 10.75 to Sierra?

Check to make sure your applications are compatible.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

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.

If you mean Sierra, you can upgrade directly. If needed, use the link below.

If you mean High Sierra, you need to download El Capitan or Sierra first.

(155082)