All of the satellites were launched to the same temporary orbit (parking orbit) via a special dispensers which released one satellite every 100 seconds.
Over the next several weeks, the satellites will undergo qualifications at the temporary orbit. Afterwards each satellite will be moved higher to various final orbits -- replacing existing Iridium satellites on a one-to-one basis. There's enough propulsion in the satellites to move them in and out of (and maintain position at) various orbital positions.
Eventually they will also place spare satellites at parking orbits.