How do you change the nation’s flag?

You can't do it on mobile. On PC you can open your country info by clicking on your country flag in the top left corner. Then download an image and click on your country flag and upload it. if you want to add a Newspaper emblem, then select the flag beside your coutnry flag (for Comintern doctrine it will be USSR flag, for Pan Asian doctrine it will be the IJN flag, etc etc). Upload an image 1:1 and enjoy newspaper emblem