Remove a package (upday) from stock theme
Sorry, really a stupid question!

Actually the problem was that I could not find the correct package name for Upday. After a lot of google digging I've been able to discover the mistery.

The package is de.axelspringer.yana.zeropage (so where is "upday").

a normal find / -iname '*axelspringer*' (followed by rm ...) did the trick.