HTC released the Marshmallow update for Sprint Desire 626s recently, which the device truly deserved. With 1.5GB (1GB in some regions) of RAM onboard, the Desire 626s is quite capable of running 6.0 update.
Now, we also have the firmware file, also called RUU (ROM Update Utility), with us now, using which you can upgrade your Desire 626s to 6.0 effortlessly.
The Marshmallow update comes with version 2.27.651.6. So, to be able to use RUU, the only you need is version lower than that.
How to: Well, simply download the above RUU file (exe format it is), and after taking necessary backup, and making sure battery is more than 35%, run the RUU file on your Windows PC. Let it do its work, which takes time, until you presented with a pop-up window. Now, simply follow the instructions on-screen to install Sprint Desire 626s Marshmallow update, version 2.27.651.6.