In this tutorial I show you how to update your firmware(OxygenOS) to latest and install custom ROM on a rooted OnePlus 5T. Why do this? Well, a lot of custom ROMs like Havoc ROM I am installing here does require you to have installed the latest factory firmware(OxygenOS) at least once before you can even install the ROM. This is case with most newer custom ROMs.
If you are already rooted on the OnePlus 5T and have not updated your firmware in awhile because you were on a custom ROM, you can use this method to update the firmware to latest and be able to install newer custom ROMs. Now, if you have just rooted your phone, you probably can skip this step of installing latest firmware and go straight to installing the ROM with TWRP recovery. Otherwise, follow along and let’s update your firmware first!
Also, this is how you can unroot and re-install stock firmware, you can simply stop after booting the stock firmware then your phone is running stock firmware again without root.
All the required files are over on Havoc ROM Review page(scroll down).