If you want to advance past using your Samsung Galaxy S6 like everyone else does with accepting over the air software updates a few times a year, you should give manual updates a try. Experienced Samsung Galaxy S6 users usually have to install software updates manually through Odin because root access and accepting OTA updates doesn't work out well. Before you can flash firmware using the Odin app, you must first enter download mode on Samsung Galaxy S6. Now that you have download mode on Samsung Galaxy S6 working, you can connect to the computer and use the flashing tool to flash firmware on your device.
The easiest way to remember why one would enter download mode on Samsung Galaxy S6 would be before you can download firmware on your device you cannot flash it and that's why you must first enter the download mode. Apart from installing firmware, download mode is also necessary if you are using one of the rooting guides with Odin. You will not find a rooting guide using Chainfire's CF-Auto-Root that doesn't require entering the download mode first just like you would do with flashing firmware.
Unrooting the Samsung Galaxy S6 is much the same as flashing an official firmware file manually if you did get root access by using the CF-Auto-Root. There are a few ways to get root access on Samsung devices and they don't always come from Odin and files to flash through Odin. For example, you might find some guides that need a custom recovery installed and then you must install the SuperSU in the custom recovery mode.
While entering download mode is necessary for some unofficial actions like getting root access, it's an official mode given to us by Samsung. You do not void the warranty or run any risk of losing data just by using the guide to enter download mode on Samsung Galaxy S6. However, those that like to err on the side of caution should backup all of the usual data, including pictures, video, text messages, settings and contacts.
The steps in the guide are available for all Samsung Galaxy S6 owners to follow, including those with or without root access, any phone carrier network, any model number around the world and with any custom or stock recovery. The key sequence we are teaching you today on the S6 does not change for anybody.
Press the Power key and then choose to Power Off the Samsung Galaxy S6.
If the keys are unresponsive, or you cannot get the display to work for any reason, you may long-press the Power key until it turns off.
Start rebooting the Galaxy S6 by holding the Power + Home + Volume Down buttons at the same time.
Do not let go of the keys until you can see the download mode is displayed on the mobile device's screen.
That was easy enough; now you all should know how to enter download mode on Samsung Galaxy S6 smartphones. If you could not get the above example to work using the hardware keys, you can always install an app from the Google Play Store.