how do i update my office '11 now that I have Catalina OS?

Buy the standalone Office X 2019 or a yearly subscription to Office 365. Visit the Microsoft website. You can also download the individual Office apps from the App Store. There is an in-app purchase for the license. The apps are freely downloadable but are just trial versions.

More information (from Microsoft) can be found here:

https://support.office.com/en-gb/article/support-has-ended-for-office-for-mac-2011-559b72b1-e045-4c73-bad3-d7f1841b9e8c