Update iOS SDK version to 8.4

问题:

I had a version of the Xcode I believe It was 6.1 with iOS SDK version 8.3.

Apple store showed that an Xcode update is available with SDK 8.4.

I updated Xcode and the new update version was 6.3.2.

I have a iPad with iOS 8.4 on it. so to run the app on it, I tried to set the Base SDK to 8.4. but, surprisingly It doesn't exists! I can only choose 8.3 with "Latest iOS" written beside it.

What can I do? And why the update said that the SDK version that is available is 8.4 since it didn't exists?!

In the updates page in Apple store app there is a list titled "Updates Installed in the Last 30 Days" and the I have Xcode 6.4 and iOS 8.4 with it. while when I open the Xcode at the welcome page it says 6.3.2 and Base SDK is 8.3!!!