Problems with installing InfluxDB and how to set the environment variable

I tried to follow this guide: InfluxDB+Grafana persistence and graphing

Here: https://docs.influxdata.com/influxdb/v1.6/introduction/installation/#configuring-influxdb-oss
it says:

Set the environment variable INFLUXDB_CONFIG_PATH to the path of your configuration file and start the process. For example:

echo $INFLUXDB_CONFIG_PATH
/etc/influxdb/influxdb.conf

influxd

I really don’t know hot to set the environment variable.
I am using the newest openhabian on a Raspberry 3b