I am using Xcode 11.5 and have uploaded a build on TestFlight. Users who have updated to iOS 14 are facing problems. Do I need to upload a new build after updating to Xcode 12 or is there any other way to fix this?
Do I need to upload another TestFlight Build for iOS 14?
Yes. If you want to test/support iOS 14.x specifically, you need to build against it, specifically with a compatible Xcode (12.x), then push that build out to testers with iOS 14.x compatible devices.