HTC is on a roll to push out Marshmallow update to their devices. Even devices in desire series are getting the Marshmallow update later or sooner.
The latest device in this table to be added is HTC Desire 820u, the China variant. The update is around 925MB in size. So it is highly recommended to be on a strong Wi-Fi connection to avoid any interruption. The ROM version moves up to 3.17.1400.4 . The updates is also expected to be coming soon to Indian variant of HTC Desire 820u. The update comes in stages. So you need to be patient in order to receive it to your device.
You can read our Big master list for Android Marshmallow roll-out for all devices from all vendors.