The EnvironmentFile bit states that the service should look in the provided file for environment variables. The - in front of the file means to not error out if the file doesn’t exist. You can read more about EnvironmentFile in the systemd man page.
The first place that I would start is changing the the path in the MongoDB config file. You can find the location of that file by looking at the ExecStart line:
In your case, the file is located at /etc/mongod.conf. Edit that file and change the strorage.dbPath location to the path you want. Save and exit from the file. After doing that run sudo systemctl restart mongod, check the status of the service with systemctl status mongod and verify if it’s running. If it is you’re all set. If it’s not you need to check the MongoDB log file (default should be /var/log/mongodb/mongodb.log I believe) to see why the service failed.