iWork '09 should still be installed.
Check your Applications folder. There should be a folder named iWorks '09.
Apple left the old versions intact after the update.
So that worked! Thank you! And then I turn on my computer this morning and TADAH it's back to Numbers 13. All my work from yesterday is off in some other universe that I can't access. This update is really getting on my nerves.
Yesterday I exported them to Numbers 09, saved them as Numbers 09, moved them to my desktop (today they are Numbers 13) and I worked on them. What did I do wrong?
"What did I do wrong?"
You opened the exported versions in Numbers 3 again (and saved them), or you opened the N3 versions (exporting a copy as v2 does not change or remove the existing v3 file).
If you opened the files by double-clicking their icons, that action willl also launch Numbers 3, which will (successfully) attempt to open them.
If you drag the v2 file's icon onto the Numbers ('09) icon on the Dock, or in the Applications folder, that should launch Numbers '09 and open the document.
Surest method is to launch Nunbers '09, then open the files via the File menu.