I installed my instance of openedx and now want to customize the name of my platform.

How to do so ?
While @hugo-lopez answer solved the problem, but you can check the current configuration before changing PLATFORM_NAME in /edx/app/edxapp/edx-platform/lms/envs/common.py. You can also check your current configuration because sometimes you forgot to restart LMS.
python manage.py lms shell --settings devstack_docker
from django.conf import settings
settings.PLATFORM_NAME
I was using Juniper release, so modifying lms.env.json not worked for me. So, I modify PLATFORM_NAME in /edx/etc/lms.yml and /edx/etc/studio.yml.
LMS_CFGenvronment variable defined on your instance? You can find that out by doing:echo $LMS_CFG. - ymas