Upgrading the Yotaphone2 from Android 4.4.3 KitKat (API 19) to 5.0 Lollipop (API 21) is highly advisable, both for improved performance and better e-ink apps. Ironically, in Lollipop the (WLAN, BT, GPS) power control widget is missing but there is one for the e-ink screen (and the Power Toggles widget may help). As usual, I got most of my information from the xda-developers forum. The internal phone name is YD206 but after upgrade it will be labeled YD201. In practice, this has no effect.
For the upgrade, you need a Windows PC, the USB cable for the phone and some hours of time.
Start with getting "developer mode". Under Settings->About Phone, tap "Build number" seven (7) times. Really. Then activate Settings->Developer Options->USB debugging. Sync your contacts and calendar one last time, we will wipe everything in a moment.
On your PC, download the Yota flasher (but no firmware yet) from the Yota FTP site; Windows only. It includes the standard adb and fastboot binaries and adds Yota-specific functionality. Install it to a location of your choice, which I will call ...\yotaflasher in the following.
Next, install the adb drivers from ...\yotaflasher\Driver\Google by right-clicking the .inf file and choosing "Install". Chances are that your Windows (mine is Win7) will complain about unverified signatures. There are multiple ways to solve the problem, and the easiest one is rebooting Windows and repeatedly pressing F8 during bootup. This will give you a boot menu; choose the last option, "Disable driver signature enforcement". Then try again.
If the .inf file insists on not wanting to be installed, there is an alternative way. Plugin the Yotaphone per USB cable, then on the PC go to Control Panel->System and Security->Device manager. There should be a ADB Interface->Android as unrecognized device. Right-click it, choose "Update Driver", then choose manual from ...\yotaflasher\Driver\Google. This did it for me. Others had more success with the standard android USB drivers.
Unplug the phone, power down, then long-press volume+power to boot in flash mode where the screen just shows "Downloading..." and nothing more. Connect to PC. Then open a command shell with admin privileges (in "Start" search field, type "cmd", then right-click the icon at the top and choose "Run as Admin"), and cd the ...\yotaflasher folder. Then issue the following commands:
fastboot devices
fastboot oem unlock
The first one should show your phone with some cryptic identifier. The second one will wipe your phone, and allow flashing new firmwares. Follow the instructions on the phone. After reboot, power down again and long-press volume+power to arrive back in "Downloading..." mode. You might need to unplug the phone for this. Now in the command shell, start:
yotaphone_flasher.exe
What appears is a simple command interface. Download the firmware of your choice (I recommend 5.0.0 EU 1.8.7), it will unpack to folder ...\yotaflasher\firmware. Then choose "Install", this takes 5 minutes, then the Yotaphone will reboot, taking another 10 minutes. As a side effect, Settings->About will have the "Update" button available again. Warning: Doing this takes an hour.
I have not experienced it, but some people had too little RAM and wound up with a non-booting Yotaphone. You might try to upload the larger firmware image parts in 512MB chunks.
fastboot.exe flash -S 512M system firmware\system.img
Also not my experience, but if some telco connectivity seems unavailable, you might try flashing the radio separately:
fastboot.exe flash modem firmware\radio\NON-HLOS.bin
fastboot.exe flash sbl1
firmware\radio\sbl1.mbn
fastboot.exe flash rpm
firmware\radio\rpm.mbn
fastboot.exe flash tz
firmware\radio\tz.mbn
Yet again not my experience, but some people had battery problems. You might try going to Settings->Developer and enable "Force GPU rendering", which restricts CPU usage a bit. Another user reported going to Settings->Wifi->Advanced and setting "Wifi frequency band" to "2.4 GHz only" improved battery life.
EOF (Nov:2016)