Sierra, macOS 10.12.6 is definitely not the most up to date version of macOS. It is 2, soon to be 3, generations of macOS behind. Numbers 6 as well as Pages 8 & Keynote 9, the only versions available in the Mac App Store, require High Sierra, macOS 10.13.6 or Mojave, macOS 10.14.x. You need to update the OS before you can update the apps.
If updating your Mac is not an option use iWork for iCloud or your iOS device to open the files & then download or share a copy. Do not choose Numbers or Pages as the file type. The best option is to choose Excel or Word as the file type which you can then open with your version of Numbers/Pages or LibreOffice or Excel/Word or a multitude of other apps.