In next phase of the rollout process, Microsoft will start making the update available to a broader range of customers. When the update is available, users will get a message on the device. All the instructions needed to install an update are here on the Windows Phone website.
Microsoft is intentionally delivering updates in batches, ramping up gradually to help ensure the process is as problem-free as possible.
Microsoft has also created a new online resource called Where's my phone update? that provides some insight into the update process.
Users will be able to see when the update is being tested and packaged, when it's about to ship, and when it's "left the warehouse."