You are correct that the device-tree is where this is configured, but the dtb file is the compiled device-tree which isn’t (directly) editable. However, in this case we don’t even need to edit the device-tree itself because the values you want to tweak are parameterized and can be customized simply by adding some lines to /boot/firmware/config.txt.
Firstly, the reference file you want to have a look at is /boot/firmware/overlays/README. This text file documents all the available device-tree overlays, and their parameters. It also documents the parameters for the base device tree, which is what we’re interested in here.
Search for fan_temp0 in that README file and you should find yourself at the start of the various fan-related parameters in the base device-tree. I’ll quote them here, but you should double-check in that file in case they’ve changed across kernel versions:
fan_temp0 Temperature threshold (in millicelcius) for
1st cooling level (default 50000). Pi5 only.
fan_temp0_hyst Temperature hysteresis (in millicelcius) for
1st cooling level (default 5000). Pi5 only.
fan_temp0_speed Fan PWM setting for 1st cooling level (0-255,
default 75). Pi5 only.
fan_temp1 Temperature threshold (in millicelcius) for
2nd cooling level (default 60000). Pi5 only.
fan_temp1_hyst Temperature hysteresis (in millicelcius) for
2nd cooling level (default 5000). Pi5 only.
fan_temp1_speed Fan PWM setting for 2nd cooling level (0-255,
default 125). Pi5 only.
fan_temp2 Temperature threshold (in millicelcius) for
3rd cooling level (default 67500). Pi5 only.
fan_temp2_hyst Temperature hysteresis (in millicelcius) for
3rd cooling level (default 5000). Pi5 only.
fan_temp2_speed Fan PWM setting for 3rd cooling level (0-255,
default 175). Pi5 only.
fan_temp3 Temperature threshold (in millicelcius) for
4th cooling level (default 75000). Pi5 only.
fan_temp3_hyst Temperature hysteresis (in millicelcius) for
4th cooling level (default 5000). Pi5 only.
fan_temp3_speed Fan PWM setting for 4th cooling level (0-255,
default 250). Pi5 only.
The syntax for customizing these is a little strange, but not too difficult. In /boot/firmware/config.txt you need to add lines with the format dtparam=PARAM=VALUE. For example, if you want to change the base temperature at which the fan turns on from 50°C to 55°C you might append the following (bearing in mind the value is in millicelcius):
dtparam=fan_temp0=55000
You can list multiple parameters in a single dtparam= line like so:
dtparam=fan_temp0=55000,fan_temp0_speed=90
Alternatively, you can use multiple dtparam= lines, if you prefer:
dtparam=fan_temp0=55000
dtparam=fan_temp0_speed=90
If your boot media is used only on a Pi 5, adding this under an [all] section is absolutely fine. However, if your boot media is going to move between different models of Pi, you may wish to place these customizations within a section limiting their effect to the Pi 5. For example:
[pi5]
dtparam=fan_temp0=55000
dtparam=fan_temp0_speed=90
[all]
# The rest of the configuration...
There’s a lot more one can do with the conditional filters, which is documented on the Raspberry Pi site.