Lenovo is known for taking it slot when it comes to updating their products to the newest software. This is the case with the new Android 6.0 Marshmallow update.
Now, the time has come for Lenovo Vibe S1. The Marshmallow is now available for download.
Your phone will pop up a notification once the new software is ready to download and install. The file is about 1.6 GB in size (as you can see from the screenshot).
You should also back up your stuff before installing the software.