[Samsung Galaxy Note 9 Questions & Answers] TWiiTCH:
First I must say, this method is for snapdragon models. If your phone is Exynos model I can't help.

What model is your phone?

First you gather the OS/firmware That you need for your model, it'll be Oreo v8.1.0 and you would want the very last one before they released pie v9.0 if you want the most updated./recent. Once you determine the firmware you need go to https://updato.com/firmware-archive-select-model and type in the search box the model of your phone. Mine is SM-N960W Canadian Snapdragon with carrier/location code being ESK whick is Eastlink mobility. You will want to know your provider 3 letter code so you don't end up with a firmware from another carrier or region.

Next since you are downgrading you'll need the modded odin. I used 3.13.1 3B whick is necessary as regular odin won't work for downgrade. Get that modded odin here https://xdaforums.com/android/software/patched-odin-3-13-1-t3762572/post75870740

Now you need stock phone USB charger cable/cord because cheap replacement cords can cause a failed transfer & maybe turn your phone into a paperweight.

Also make sure your first step is to install Samsung mobile phone drivers. Get here http://www.mediafire.com/file/d73zx1r4zc7yb0d/Samsung-USB-Driver-v1.5.63.0.zip

Install this driver on your laptop/pc & then reboot.

Now turn on developer mode menu by going to settings>about phone>software information>build number. Tap on build number a bunch of times, like ten times fast to be sure and you will see developer mode enabled. Back out to phone settings menu & scroll to bottom and tap developer options. Turn on USB debugging & make sure while in the developer menu the USB Configuration is set to MTP (Media Transfer Protocol) and not charging option. Now you can return to your home screen.

#### W A R N I N G ####1

YOU CANT DOWNGRADE TO A FIRMWARE WITH A LOWER BOOTLOADER VERSION. CHECK IN SETTINGS FOR BUILD NUMBER. COUNT BACK FROM LAST CHARACTER 5 DIGITS FROM THE END AND THAT'S YOUR BOOTLOADER. SEE IMAGE, MY BOOTLOADER IS v1 AS YOU'LL SEE. DO NOT DOWNGRADE IF Your CURRENT BOOTLOADER IS ALREADY HIGHER THAN THE ONE IN THE DOWNGRADE FIRMWARE "OREO" & MAKE SURE PHONE IS 100% CHARGED! BEFORE PROCEEDING REMOVE ALL GOOGLE & SAMSUNG ACCOUNTS OR YOU WILL TRIP FRP (factory reset protection) WHEN YOU TRY TO DOWNGRADE. ALSO REMOVE SIM CARD "important", AND DON'T CONNECT TO WIFI, HIT SKIP ON THAT SCREEN. WE CAN'T LET THE PHONE CONNECT TO ANYTHING UNTIL WE DOWNGRADE TO PIE V8.1.0 AND WE HAVE USED A PACKAGE DISABLER OR ADB CODE TO STOP FORCED UPDATES & ALSO UPDATE MESSAGES.

First open the patched odin software and insert each of your downdrade firmware files (after decompressing the firmware file you downloaded (there should be 5 files inside it). Put each file in proper spot, see image after paragraph. Its easy to know what goes where because each file has Capitol letters that correspond with the fields on the add buttons. Do not touch any other settings in odin. Now there are two CSC files in your downloaded firmware file. I recommend the one that DOESN'T say "HOME" so that you will have a fresh & clean install, the "HOME" CSC keeps your apps and settings. As long as you are backed up to microSD or Google & Samsung a clean install is best. the NON HOME CSC is what you want. Keep odin open & running from here until finish, and go to the next step.

Now put the phone into download mode. To do this press and hold Bixby button & Volume down button and power button all at the same time until you see a greenish/aqua green screen that says download mode. When you are here don't press anything, just plug in stock USB phone cord to phone, then when you plug the other end into computer you may be asked to permit connection on phone, pc, or both so select yes allow connection if this happens. Now press continue (volume up) & There will then be a rectangle box in top of odin, these will have square boxes above them. Once you have plugged in the phone & odin is running press volume up on download mode screen to continue. Once you do this you should see one of the rectangle boxes light up and say something like connected or com:2 or along those Lines. Only thing that really matters is that one of the boxes will now have color & or info in it.

Next in odin on pc hit the start button if you've made it this far trouble free. The process will begin, it takes maybe 7-10 minutes to write firmware to phone but if it seems like it's stuck or failed but does not give fail message give it 45mins, there isn't a guarantee on time it takes. Once you see a green square saying (pass) inside odin your free 2 unplug phone.

Do not let phone connect to wifi or mobile data until The Offending Package is disabled using ADB commands or Package Disabler. Get the disabler onto your phone with no connection by putting it on micros card first & sideload it. Hard to pull off, I used ADB method. Offending Package is... com.wssyncmldm that's the one causing your phone to auto update.

Make sure you wait for the phone to boot after downgrading & keep in mind it can take 30 minutes or more in some cases. Leave it be, it'll boot if you did it correctly.

Cheers!