Hello,
The best way to answer you about your question demands know what operational system your cell phone have.
Considering your cell phone an Android SO, the simpliest manner to downgrade is just download the apk version from apkmirror, for instance.
There is some modificated apks made from DJI GO 4 4.1.15 (DJI GO 4 mod, DJI GO 4 ITA) as well, helping you to get FCC frequencies and some features like a mavic.
But, I would like to suggest you to install and use Litchi instead DJI GO. For me, this aplication have worked out better then DJI (more stable, using both wifi conection or OTG cable conection, including cheap VR possibility)