I have tried with this but couldn't solve my issue - Ineligible Devices section.
I m getting an error The Developer Disk image could not be mounted error with iOS 8.4 device
can anyone please help me how can I run iOS 8.4 version device with Xcode 6.3.1
or updating the Xcode version is only the solution
Thank you all in advance
You will need to upgrade to Xcode 6.4 in order to support iOS 8.4 development -
From the Xcode 6.4 release notes -