How do I change my macOS user account UUID to stop Apple tracking me?
Firstly, the premise behind your question is false on two grounds:


The UUID isn’t actually a UUID in the real sense. - What I mean is that it isn’t a universally unique identifier that, for anything more than practical purposes, is unique. See universally unique identifier for more info. So, while your user account UUID looks the same as a real UUID because of its 32 hexadecimal digits in the 8-4-4-4-12 pattern, it’s not actually a UUID in the true sense of unique. That is, it's not like there's a central authority ensuring that the possibility of two UUIDs being the same is zero!
Apple does not use the UUID to track you - Despite occasional rumours that Apple uses this to track the activity linked to the Admin account of a Mac, there is actually no evidence of this. (This is also confirmed by 1 above - i.e. since it's not a real UUID it can't really be used for tracking purposes). Instead, this UUID is used by Apple’s Open Directory to manage information about your Mac user accounts and provides them with access to network resources. See Apple Open Directory for more info. 


Secondly, since you’ve got an iMac Pro, you actually can’t change the UUID. More specifically, iMac Pros all shipped with macOS High Sierra as their original OS, and as of this version of macOS the option to Refresh or Change the UUID is no longer available like it was in earlier generations of macOS (albeit hidden away). Why? Because too many users were doing this to resolve issues and ended up causing themselves major grief instead. 

Finally, you may want to familiarise yourself with Apple’s stance on privacy:


Our approach to privacy 
Manage your privacy 
Transparency report 
Privacy Policy 


If you’re still concerned, you can also contact Apple directly about privacy issues.


WARNING for users of macOS up to and including Sierra - Do not refresh/change your UUID. Doing so will almost certainly damage your account and, in worst case scenarios, prevent you from logging in.