I recently updated Xcode 6.3.1 to Xcode 6.4. Updates went well and I installed it successfully.
Problem: When I open it after installation, it was the same version as before. Xcode 6.3.1
P.S: Image is attached. Appstore is showing that xcode is updated to 6.4 version
I think it Might be the bug in xcode 6.4 update .... Try to install again the 6.4 after uninstalling the older version.