First of all:

WARNING:
This is an early build and not a stable build. It's NOT recommended for a daily use yet. Front-Facing camera is not working, AR-emoji will crash.
Only install if you know how to use Odin, and install the UPDATE.ZIP via recovery.
Note: it'll be difficult to revert back to Oreo!


NOTE: this is for SNAPDRAGON-devices only, model SM-G965U -> NOT for the EXYNOS-models (sold in the most parts of the world)

First you've to download some files:

1. Odin 3.13.1, which can be downloaded here: [ Login above or register to see download links. ]

2. You need a specific build G965USQUS3ARG8_TMB on your device: In it, you will see six files. In Odin, you will see 5 categories, though you will only use 4. This is a generic update, not one specifically for T-Mobile. It will keep carrier branding though..
This file can be downloaded here: [ Login above or register to see download links. ]

3. Next download the UPDATE,ZIP file, which can be found here: [ Login above or register to see download links. ]

WARNING:
This is an early build and not a stable build. It's NOT recommended for a daily use yet. Front-Facing camera is not working, AR-emoji will crash.
Only install if you know how to use Odin, and install the UPDATE.ZIP via recovery.
Note: it'll be difficult to revert back to Oreo!


Be sure, your S9+ has 100% battery (for safety); now we have downloaded all the files, we can start.

1. Put an SD Card in your phone, copy the update.zip to the SD Card.

2. Open the Odin files. The zip will be named G965USQUS3ARG8_TMB.zip. In it, you will see six files. In Odin, you will see 5 categories, though you will only use 4. This is a generic update, not one specifically for T-Mobile. It will keep carrier branding though.

3. Put your phone into Odin mode by turning it off then holding Power + Volume Down + Bixby button.

4. Open Odin and put the corresponding BL, AP, CP, HOME_CSC (ignore CSC), but nothing in Userdata.

[ Login above or register to see download links. ]

5. Once in download mode, connect your phone to the PC, preferably using the original USB cable you got with your phone. Odin should detect your device and a BLUE box will show up with the COM port number. (Be sure, the SAMSUNG-drivers are installed on your PC!)

6. Do NOT tick any extra boxes. The only options to be ticked are F.Reset Time and Auto-Reboot.

7. Click the start button, wait for Odin to say 'PASS' in a GREEN box. Once the flash is complete, your phone will reboot, and you can disconnect your phone from the PC.

8. After you do this, wait about 5 minutes and then shut off your phone and reboot into recovery by holding Power + Volume Up + Bixby.

9. Use the volume buttons and power to select “apply update from SD card.”

10. Use the volume buttons again to select the update.zip file and the power button to select it. The update will then start. The update can take between 2 to 10 minutes.

11. Once the update finishes, we recommend you factory reset. This can help out with stability issues.

Android Pie will be on your device now.

WARNING:
This is an early build and not a stable build. It's NOT recommended for a daily use yet. Front-Facing camera is not working, AR-emoji will crash.
Only install if you know how to use Odin, and install the UPDATE.ZIP via recovery.
Note: it'll be difficult to revert back to Oreo!


NOTE: this is for SNAPDRAGON-devices only, model SM-G965U -> NOT for the EXYNOS-models (sold in the most parts of the world)

NOTE 2: SamMobile is not responsible for any damage caused by using the files