[OnePlus 6T  Guides, News, & Discussion] mxz55:
This guide exists next to [GUIDE] Extracting the latest firmwares from ROM update package and they supplement eachother.

General word about firmware updates:

A lot of people know of the importance and benefits of keeping your device firmware updated, just like you keep your custom ROM updated. If you don't keep them updated, you might not get the best out of your custom ROM (like: more bugs, system stability and performance problems/less speed.. also, vendor components, e.g Qualcomm and other chips, have security patches in the firmware.. "Vendor patch level" vs Android "Security patch level").

Besides incremental firmware updates from each OxygenOS release with a small changelog, too many people are missing out on huge jumps such as that they are running an Android 11 custom ROM, while their firmware was never updated so it remained a firmware originally shipped with an Android 10 (or earlier) OxygenOS release. Staying behind on huge jumps like these, causes the biggest list of problems with stability and performance. Therefore it is advisable to keep your firmware updated, and note that even small incremental updates (based on the same Android version) tend to give performance, security and stability benefits.

Prerequisites

For flashing the latest firmware, you need to have the firmware files (NOT the ROM update .zip file.. but a bunch of .img's) first. You can get them in multiple ways:

- Look for someone on XDA that has shared them, after they extracted the firmwares from the ROM update .zip file. This saves you time, but if you want to do it by yourself then first follow the "Extract firmwares" guide. An example of a reason for doing it yourself is: you want to be sure of the authencity, completeness and integrity of firmware files (that is everything but guaranteed if you DL it from another user)

- Do it yourself, as described above, and for this use the guide linked above

So after this stage, you have the firmware .img files, as required to proceed:

GUIDE STARTS BELOW THIS LINE

1) Get adb and fastboot from https://dl.google.com/android/repository/platform-tools-latest-windows.zip

2) Extract the above (adb and fastboot) to a folder named "FW update". We assume that's the location all of the extracted firmware (.img) files are, like they would be if you followed the "Extract firmwares" guide to get them

So, now that your adb and fastboot are in the same folder as all required firmware files, you can proceed.

3) Actual flashing instructions, a very simple process

1) Boot to Recovery
2) Enter Fastboot (e.g for LineageOS recovery: "Advanced" > "Enter Fastboot")
3) Connect phone to PC with USB cable
4) Open cmd (command prompt) in your adb / fastboot binaries location folder (in cmd, enter cd [location of your "FW update" folder.. or alternatively copy cmd.exe from system32 to "FW update" folder & open it there),

5) Start flashing..
Enter commands 1 by 1:

fastboot flash --slot=all abl abl.img
fastboot flash --slot=all aop aop.img
fastboot flash --slot=all bluetooth bluetooth.img
fastboot flash --slot=all cmnlib cmnlib.img
fastboot flash --slot=all cmnlib64 cmnlib64.img
fastboot flash --slot=all devcfg devcfg.img
fastboot flash --slot=all dsp dsp.img
fastboot flash --slot=all fw_4j1ed fw_4j1ed.img
fastboot flash --slot=all fw_4u1ea fw_4u1ea.img
fastboot flash --slot=all hyp hyp.img
fastboot flash --slot=all keymaster keymaster.img
fastboot flash --slot=all LOGO LOGO.img
fastboot flash --slot=all modem modem.img
fastboot flash --slot=all oem_stanvbk oem_stanvbk.img
fastboot flash --slot=all qupfw qupfw.img
fastboot flash --slot=all storsec storsec.img
fastboot flash --slot=all tz tz.img
fastboot flash --slot=all vendor vendor.img
fastboot flash --slot=all xbl xbl.img
fastboot flash --slot=all xbl_config xbl_config.img

fastboot reboot

Done, now your phone will boot up with the updated firmwares.