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Backup everything on your phone prior to rooting or flashing the CM13 Marshmallow ROM, which requires a Factory Data Reset before installing. See, Backup! Backup! Backup! NOW!

To root your phone you will need to flash a rooted CF-Auto-Root kernel for your model of phone, via Odin. See, [SM-G900*] CF-Auto-Root

You will also need to flash a custom recovery, such as TWRP, for your phone. See, TWRP for Samsung Galaxy S5 Qualcomm

Make a note of your firmware and Product code in case you need to revert to stock. See, #1.8 of 41 Galaxy S5 models - Dummies Guide

Make a nandroid backup to your external SD card using TWRP.

Yes, by downloading and flashing your stock Samsung firmware, mentioned above or by restoring your nandroid backup.

Yes, as it will trip the Knox Security flag. Not even by reverting to stock firmware can you un-trip it.

The best place to post for rooting questions is on the Root - Samsung Galaxy S5 section of the forum.

For links and details of the Marshmallow, CM13, CyanogenMod ROM, see, Samsung Galaxy S5 gets Marshmallow

These ROMs are released nightly and are beta test ROMs and require some amount of experience and a lot of reading of the various threads before flashing.

Read, read, read, everything before asking questions, rooting or attempting to flash CM13 and/or gapps.



ironass motto... "Flash in haste... repent at leisure!"