how do i get old microsoft word back after upgrade to catalina

Microsoft only supports the current Microsoft 365 and Office 2019 for Mac suites on Catalina. It also supports the 64-bit Office 2016 for Mac which they have indicated will be retired in Fall 2020.

If you upgraded from macOS Sierra or earlier, and even if you had a last Time Machine backup from that installation, you would not be able to activate a reinstallation of Office 2008/2011 because Microsoft has turned off those activation servers. If you upgraded from High Sierra or Mojave, you may be able to go back by booting into Recovery, and choosing to Restore from your Time Machine backup, which would preserve your personal files and installed applications, though you may still need to reinstall your version of MS Office (if 2011).

Paragraph 2 is a royal PITA and you would be better served moving forward with a new 365 subscription, or single-purchase Office 2019 for Mac. Either will open your existing MS documents. And has BDAqua has mentioned, there is the free, 64-bit LibreOffice Suite which is a Microsoft Office clone.

Microsoft Office 2011 and older will not run on Catalina. You can upgrade to Microsoft Office 2019 or Office 365.

Microsoft Office 2019 and 365

depends on what Office version do you have. Some of them are not compatible with Catalina.

Look into the Free LibreOffice, it handles MS Office files…

https://www.libreoffice.org/