Microsoft recently halted the Windows Phone 8.1 Preview for Developers due to a bug which caused some handsets encrypted with Bitlocker to crash when installing an update. This halt has not only postponed the next preview update, but it has also postponed ALL updates for those on the preview.
What does this mean? Well simply put, if Lumia Cyan is available for your device, and you're running the Developer Preview, you will not be able to obtain Cyan until you downgrade to Windows Phone 8.0. Yep, you read that correctly. You're going to have to downgrade to upgrade.
Once you downgrade, you'll be prompted to install Windows Phone 8.1 + Lumia Cyan, if Cyan is available for your device.
If you're unsure how to downgrade, you're going to need to download the Nokia Recovery Tool which will allow you to downgrade to Windows Phone 8.0. You can download that client from the download link below. Once you've got the client, connect your phone to your PC, make sure you've backed up your device and it has been recognized by the client.
If your device is showing up, just hit the install button. It will download Windows Phone 8.0 for your device, and then a set of on-screen instructions will appear which will tell you what to do in order to flash it correctly.
Microsoft hasn't commented on an ETA for a fix for Preview users, so if Cyan is available for your device, your best bet would be to downgrade.