Android One was one of the first to receive Android 5.1 Lollipop but it wasn’t until now that the OTA was captured. If you wish to upgrade your Android One device to Android 5.1 Lollipop without losing any data, you can do so by installing the OTA .zip file. (build number LMY47O)
Keep in mind that you dont have to factory reset your device before flashing, as this is an OTA update you can just flash it & get updated to Android Lollipop 5.1 even without losing any data.
Please make sure to download the file for your specific device as installing the wrong one could cause problems.
Micromax Canvas A1
Karbonn Sparkle V
Spice Dream UNO
Symphony Roar A50