[Boris The Spider]

In the FMB after selecting the aircraft you open the "Properties" tab.

Here "None" has been selected as the aircraft's squadron. This will place no regiment emblem on the plane:


Here, 4rd (lol) GvIAP KBF has been chosen so that regiment emblem would be displayed:


To change the regiment emblem just open the mission in the FMB and change it in the "Properties tab. To remove it, choose XXXArmy- .

You can also change the regiment patch in the Aircraft Customization menu as shown here:


Hope that helps.