The time has come, Windows phone users.

Microsoft said Thursday it is rolling out the highly anticipated Windows 10 mobile update for select existing handsets.

Starting today, Microsoft said the Windows 10 upgrade will be available on a number Lumia smartphones running Windows Phone 8.1, including the Lumia 1520, Lumia 930, Lumia 830, Lumia 735, Lumia 730, Lumia 640 XL, Lumia 640, Lumia 638 (1 GB RAM), Lumia 636 (1 GB RAM), Lumia 635 (1 GB RAM), Lumia 540, Lumia 535, Lumia 532, Lumia 435 and Lumia 430.

Though the roll out for existing devices starts today, Microsoft said the availability of the update may vary based on a user’s market and operator. Users can find out when the update will be available on their device using Microsoft’s upgrade advisor app.

The upgrade will bring new features to existing Windows smartphones, include improvements to Microsoft Outlook and Office, alignment of Cortana with Windows 10 desktop and availability of universal apps that cross devices.

However, Microsoft said the update will also modify or remove some features currently on Windows devices, including notifications for missed calls, messages or emails on contact tiles and Cortana’s ability to search for apps, setting, email, text messaging, contacts and QR Codes. Microsoft also said its “Data Protection Under Lock” feature, which allow for the encryption of email data and attachments and requires a pin lock key to decode the content, is not available on Windows 10 mobile.

While substantially smaller than the 3 GB file required to install Windows 10 on a PC or tablet, Microsoft said the mobile update file is still about 1.4 GB. Microsoft said users will need a Wi-Fi connection to download and install the upgrade.

If you’re running low on storage space before the download, though, beware. Microsoft said the upgrade will use “some” device storage space and warned users “may need to remove unneeded files from your device to complete the upgrade.”

For those who would are looking for a new device, Microsoft said the new Lumia 550, Lumia 650, Lumia 950, or Lumia 950 XL devices come with Windows 10 straight out of the box.

Windows 10 mobile is also available on the Alcatel Fierce XL, which T-Mobile introduced as its first Windows 10 offering last month.

Read More: Microsoft Reveals Cutoff Date for Windows 10 Mobile Support