How do I update pages?

Any update of Pages from v5.6.2 on El Capitan to current versions in the Mac App Store will require that your Mac can (and has) upgraded to macOS High Sierra or later. That Late-2009 iMac can upgrade no further than High Sierra.

There is a dedicated Pages for Mac Support Community available to you.

A Time Machine backup before you upgrade. Set that drive aside and do not use it for the upgraded operating system, in case you need to revert. Get another Time Machine drive for exclusive use of your upgraded operating system.

The upgrade will not touch the contents of your home directory, but there are things you need to do prior to the upgrade:

1. Check that your third-party applications, printer, and scanner drivers are supported on High Sierra by checking the vendor sites.
2. Boot your Mac into Recovery (⌘-R) and use Disk Utility First Aid on your internal boot drive to ensure it has a green bill of health.
3. Ensure that you have at minimum, 20+ GB of available space on that internal drive. Click on your Desktop boot drive icon and press option+command+i to see Available.

When I upgraded my mid-2011 Mac mini from El Capitan to High Sierra, it did not result in a significantly slower machine, though you can count on every new release of macOS introducing more processes and thread usage. Nature of the beast. As long as you have 8GB of RAM, and do not attempt to run more applications than your RAM supports, things should be fine.

Link to check applications compatibility.

Application Compatibility