If you scan the QR Code from the DJI website on an IOS device, your going to go straight to the App Store to download version 1.4.8.. There is no way to revert to a previous version.
There are perhaps some utilities that if you had them, would have backed up previous versions if you had done that, and might have re-installed them, but not sure how well they work and you'd have to have had already done this, and they are not free.
You did not mention iPad model , you may have an older iPad, that may not longer have the power needed to effectively run Fly, check Settings->General->About for model. If you have an iPad 5 should run it just fine, however if you have that model, and still have issues, you may need to do some cleanup, check free space. Perhaps try a clean install of Fly, or do a clean install of IOS on the iPad and install Fly and see how it runs.