Although there’s no release info but a beta can be released by Microsoft on the BUILD Conference. More news will be provided by Microsoft on BUILD Conference. Windows 8.1 may release sometime later this year.
A leaked screenshot few hours ago indicates that Windows 8.1 build 9374 is renamed to ‘Pro Preview’. So the name of this build is ‘Windows 8.1 Pro Preview’ according to the leaked screenshot. This build string of this version is 6.3.9374.0 (fbl_partner_out15.130329-2355). It was compiled on 29/03/13.