my computer is too old to update to macOS big sur , what do I do?

If the applications require Big Sur your only options are to replace your Mac with one that can run Big Sur or find alternative apps that can run in High Sierra.

Continue using it as you did before Mojave, Catalina and Big Sur were released?

What Apps?

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/