How to download software update?

I have looked everywhere, but I can't seem to figure this out. I have macOS Sierra version 10.12.6 on my Macbook. I need to update it in order to download Adobe Photoshop, but I cannot find where to update my system. There is no "software update" in my system preferences, and it does not show up as an update in the app store. I am just very confused at this point, and any help would be appreciated.