how do I update my Microsoft Office/

Depends on the version of MS Office you have. But, only an updated Office 2016 or later will run on 10.15 Catalina, so if you have an earlier version (e.g. Office 2011) you'll need to buy Office 2019 or subscribe to Microsoft 365.

If you are on the Microsoft 365 subscription, you will be offered updates to your Microsoft Office products. On the other hand, if you purchased standalone products, you will to update them by going through Microsoft.