Extract the .NBH file from your prefered ROM upgrade: In windows use winrar (right click) to extract the contents of the exe file. In Linux / MacOS use cabextract (or you can also use winrar via wine).
Rename the .NBH file to TRINIMG.nbh and copy it to the root folder of your miniSD card.
With miniSD card inserted on the phone, start the bootloader (hold Power button on right side and Camera button at same time and stick with stylus in RESET on bottom).
You should see the screen pictured on the left, confirm flashing on screen by pressing the power button.
Flashing might hang after some time but this is normal. Just wait. Complete flash takes about 30 minutes, so check your battery or connect to charger before the flashing process starts.
When flashing completes soft-reset your device.
Make sure your SD has FAT32 and has no label. There have been reports that if SD has a label the bootloader can not access the TRINIMG.nbh
Make sure you are using a miniSD and no microSD with adapter.
For normal (non preproduction) devices keep in mind that if the CID does not match the flashing will not work. The CID for each ROM is listed on the Trinity Upgrades page. You can see your device's CID using info 2bootloader command or using GetDeviceData.exe
SuperCID devices are able to flash any NBH image using this method.
cabextract