Lenovo today has started rolling out Android 6.0(Marshmallow) Update for Vibe X3 Smartphone in India. This update is available Over-The-Air(OTA). Users can check for the update by just going to Settings > About Phone > System Updates. The size of the update is 1711MB. The version of the update is X3a40_S217_160623_ROW. The official change log of the update is not available at this time. However, this update will fix many bugs and will increase device stability. It is advised to take backup of data before upgrading.
If you have already updated your device do tell us your experience in the comments below.