If you haven't emptied your trash or recycle bin, the old version should be still there. You can copy it to a safe place, delete the 2.42 off of your mobile device, and reinstall the older version.
I have a Mac, so it may be different for Windows, but here is how I do it.
After you install the new version, you can find the old version in the trash can. I moved it out of the trash and temporarily onto my desktop.
I deleted the 2.42 app from my iPhone, then clicked on the old version on...
I can no longer sign in to my DJI account on the app since I did the update. It tells me I need to enter a valid email address. It's the same one I've been using the whole time.
Time to go downgrade back to the previous iOS version and see if that helps.
I've deleted the DJI Go app and reinstalled it several times. I still have the issue. Like I said, everything works fine on iOS 8.4.1. I am hoping that when iOS 9.1 comes out later this month or early next month, this will resolve itself.
On iOS 9 on my iPhone 6s Plus, I get "Map Loading" on all of my recent flights. On my iPad Air 2, which still has iOS 8.4.1, all of my flight data is correct, with no "Map Loading" errors. I am sticking with iOS 8 for now.