[Sprint Samsung Galaxy S6] koop1955:
I wanted to update my phone from Software version/Baseband version PF4 to the latest Software and Baseband version PG1, but I wanted to KEEP EVERYTHING ELSE. All my apps, my ROM and my Kernel. I was able to do this in less than one hour by following these steps.

Necessary files first:

1) ODIN file. You want to have a folder with the Odin .exe and other files somewhere convenient, your desktop is fine.

2) Latest TAR file available. Currently, that is PF4. The file should end in .md5.

3) Latest CF-Auto-Root. The file should end in .md5.

4) Latest Recovery (TWRP) TAR file. The file should end in .tar.

All of these are available from thread here: http://xdaforums.com/sprint-galaxy-s6/general/guide-links-files-update-root-restore-t3366862

All Put the PF4, the CF-Auto-Root and the TWRP file in the folder where Odin is.

For convenience, I also recommend installing Minimal ADB. Grab that here: http://xdaforums.com/showthread.php?t=2317790

Install the Minimal ADB. Run that and make sure that it sees your phone. Plug your phone in and type "ADB devices." Look at your phone and accept the security prompt if that comes up.

Once you have Minimal ADB installed the commands that you will use are "ADB Reboot Download" and "ADB Reboot Recovery." These just simplify things.

Is your phone Rooted with TWRP Recovery? Great, we can get started! If not, open ODIN. Flash CF-Auto-Root to get your phone rooted. Then flash the TWRP TAR file. And then you are ready to begin.

Make sure that you have Developer options selected and OEM unlock selected. Really. Make sure. Otherwise you will get the FRP lock.

1) From ADB, type "ADB Reboot Recovery." This should take you to TWRP. If it doesn't read up ^^^^ because you missed a step.

2) In TWRP, go to Backup. This should be the default but if not select Boot, System and Data. Make that backup, which will get stored in TWRP/Backups on your phone. Within that folder, you will see a folder with a long name with lots of numbers and letters that identifies that this backup is for your phone. I generally copy the whole Backup folder to my PC, just in case my phone gets totally wiped I can still restore it. It takes a few minutes and better safe than sorry. Backup done, you can go to the next step.

3) In ADB, type the command to put your file into Download mode. ("ADB reboot download.") Start ODIN. Click on the AP button and select the PF4 file. Let it load, it takes a while. Then click start. This will take few minutes, maybe 10, then your phone will restart back to stock. Again you are going to have to wait for a bit.

4) Let your phone finish rebooting. Once done, you can go to Settings -> System Update -> Download updates manually. You should have an update to PG1 available. Install that and again, your phone will reboot. Once that is done, check your Software version/Baseband version and they should both say PG1.

5) Again, use ADB to send your phone back to download mode and back to Odin. Select the CF-Auto-Root and Start. This should go much faster.

6) Back to Download mode and Odin one last time. Select the TWRP file and start.

7) In ADB, type "ADB reboot recovery".If you have done everything right, you should now be in TWRP.

8) I like to get rid of any bloat that may have been installed with the Stock. I go into Wipe and do both a Factory reset and then in Advanced wipe Caches, System and Data. Then go back to the TWRP main screen WITHOUT REBOOTING.

9) Click Restore and restore the backup that you did at the beginning.

Your phone is now updated to the latest Software and Baseband versions but with all your apps and data as it was before you started.

This literally took me 45 minutes today and that only because of the long rebooting times.

Have fun!