Hi Vincent! You might not be able to change aliases in a published data source. I know this is the case with Tableau Server Data Sources. You could try saving a local copy of the data source, changing the aliases, and then re-publishing the data source.
It seems like it would make sense for me to be able to change aliases in a published data source that I'm the owner and creator of. Well, it's not a big deal for now. I made 2 calc fields to serve as a patch:
IF [email field] = 'firstname.lastname@example.org' THEN 'No Email' END
IF [State] = 'TX' THEN [replacement calc] ELSE [email field] END
I use the Patch calc in the display, and it works pretty well. Still, moving forward I'd like to have control over my own data source :-/
Yeah, I agree, it's pretty weird that you can't just edit your own published data source on the fly. But as far as I know, that's just how it is. It's pretty easy to create a local copy (by right-clicking it) and then republish, though.