A source with inside knowledge of Microsoft's plans say that a developer version of the updated OS will be available starting in April. Those with an App Studio account will be able to access the Developer Preview of Windows Phone 8.1.
According to the report, at the same time that Windows Phone 8.1 will be pushed out, a number of new handsets will be released, to take advantage of the improved operating system. The report didn't mention any specific brand or model.
Even if Microsoft releases the update in the July-August period, many won't see it for weeks afterward as the revised OS goes through carrier testing.
source: WPBar (translated) via WMPoweruser