When a TRMNL device is in "boot mode," it will accept incoming data from a USB-C connection. This data is mostly likely a firmware update.
First, turn your device off for 10 seconds before following steps below.
Boot Mode - Basic
From our flashing page:
Turn off device
Plug in device with USB-C
Hold down "boot" circular button on back of device
While holding down button, switch device "on"
Let go of "boot" button
Boot Mode - Advanced
If your TRMNL is being stubborn, the process is thus:
Turn off device
Hold down "boot" circular button on back of device
Plug in device with USB-C, while still holding down "boot" button
While still holding down button, switch device "on"
Let go of "boot" button