how do I now use MS word after catalina update?

What version of Microsoft Office did you have?

Office 2011 and earlier will not work in Catalina. This has been warned for the past 2 years, and the Catalina installer should have further warned you about it before installing it.

Office 2016 has an update: https://docs.microsoft.com/en-us/office365/troubleshoot/administration/office-2016-for-mac-64-bit-upgrade

365 and Office 2019 should work without issue.

If you have Office 2011 or earlier you will need to buy a new version of Office to work on Catalina: https://products.office.com/en-us/compare-all-microsoft-office-products?activetab=tab%3aprimaryr1

Alternatively you can try out free alternatives such as Libre Office or Open Office or even Apple's own Pages, Numbers and Keynote Apps.

Due to Catalina's strict 64-bit application requirements, you will need to make a decision whether you want to upgrade to the current Office365 (subscription), Office 2019 for Mac (single purchase), or use an MS Office alternative suite (all of which are 64-bit solutions). Your existing Office documents will open in any of these. Those alternatives to Microsoft could be:

• LibreOffice (Free)
  • MS Office superset, so if you use those features not in MS Office, there will be compatibility issues with MS Office users.
• SoftMaker Office or SoftMaker NX (subscription - less than MS)
  • Native MS document support. No conversions.
  • Free trials

The old 32bit software is no longer compatible with Catalina's 64bit operating system.

Warning were issued for 2 years, if you missed them well sorry. In short, you need to buy either Office for Mac 2019, use Office 365 or use another alternative.