The new Android Security Patch for April is now live, with both OTA and factory images up for download for all Pixel devices and even a few Nexus devices. If you have any device in the Pixel or Pixel 2 family, including the Pixel C, you'll be able to download both OTA and factory images. Those still clinging to the slightly older Nexus 5X and Nexus 6P can also get this new update straight from Google in either form. No matter how you get the update for a supported device, at this stage, you'll need to install it manually. Expect the actual OTA updates for supported devices to roll out within the next few weeks at the latest.
This month's patch brings over 100 fixes, most of which come from Qualcomm and affect the closed-source components of the code used to run Android on the company's Snapdragon mobile chipsets. Of the total fixes in this patch, only 23 do not pertain to Qualcomm hardware and software. Among these is a critical fix for Broadcom components that could allow an attacker to execute arbitrary code remotely and gain privileged access, along with patches in the system and media framework that address that issue on a deeper level, preventing attackers from using exploits outside of Broadcom hardware to perform the same attack. There were also fixes in the Android runtime and system framework that prevent attackers from bypassing user interaction requirements to gain privileged access to a remote device.
Installing these updates manually happens the same way as with previous updates. You'll need to have a PC with ADB and fastboot set up, then either use ADB to sideload the OTA zip file of your choice, or flash the applicable image with the fastboot flash-all command. Note that using the factory image method may require unlocking your device for flashing, a move that will need to be reversed before you can send the phone in for warranty for any reason, and is generally recommended for security purposes. Users who don't want to hassle with all of that can simply wait for the OTA updates to roll out, which should happen within the next few weeks at most. Users who are a bit too impatient for that or want to pull the files and analyze them will find the buttons below convenient.