Change the Default_root_dir in YAML File

[yxteo2]

Hi guys, how can I change the root directory in YAML file.
I try to:

datamodule.trainer.default_root_dir = THE DIRECTORY I WANT

but I got this

╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮ │ in <module>:1 │ │ │ │ ❱ 1 datamodule.trainer.default_root_dir │ │ 2 │ ╰──────────────────────────────────────────────────────────────────────────────────────────────────╯ AttributeError: 'NoneType' object has no attribute 'default_root_dir'