If you can’t receive the OTA update notification on your phone, then you can use TWRP recovery to flash the beta Android 6.0 Marshmallow firmware on your Samsung Galaxy S6 or Galaxy S6 edge. The normal way that Samsung wants you to beta test the new Marshmallow is by downloading and installing the Galaxy Care app and then joining the Galaxy Beta program.
But even if you join the beta group, it may take up to 24 hours for Samsung to allow you to download the firmware. If you are one of those who cannot wait, then you can also manually flash the beta Marshmallow firmware on your Galaxy S6 or Galaxy S6 edge.
In order to do so, you need to BTU UK firmware installed on your phone. Besides that you need the update.zip file which contains the Marshmallow firmware. We are going to install this file using TWRP recovery or a stock recovery.
How To Install Update.zip Marshmallow Firmware on Galaxy S6 & S6 Edge
You need to flash the BTU firmware if you are running on another firmware. Check out this guide to learn how flash the firmware. Now that you have installed the firmware, you need a custom recovery such as TWRP and the update.zip file.
1. Turn the phone off and boot into recovery mode by pressing the volume up, home and power keys at the same time.
2. In recovery, go to ‘Update ZIP from SD Card’ or ‘Install’.
3. Go back, wipe cache and reboot the phone.
This will flash the new Marshmallow firmware on your Samsung Galaxy S6 or S6 edge. Keep in mind that this build is still in beta and not completely stable.