how do I download Catalina OS upgrade?

This Link will go to Apple Site and present various macOS versions to download including Catalina. Be sure to open this links Only In Safari as FF / Chrome will not function. Caution: there are unconfirmed reports that Apple Servers are not playing niece and the download from Apple Store will fail. It is also reported that they ) Apple ) have been advised, so if it does work now, try again in some hours as it may be fixed by then.

iTunes ?? iTunes was split into Music, TV, Podcasts in Catalina. I know it will open Apple Apps Store unless, as perviously mentioned, Apple as not yet fixed things. Can only suggest trying again at a later date or time.

How to get old versions of macOS - Apple Support

Ok, just tried and yes, can not connect to Apple Store - so Apple must be still trying to fix things.

FYI -I Reported the issue to Apple - the best that can be done for now. Patience please for it to be fixed.