You can customize DC/OS Spark in-place when it is up and running.
Go to the DC/OS GUI.
Click the Services tab, then the name of the Spark
framework to be updated.
Within the Spark instance details view, click the menu in the upper right, then choose Edit.
In the dialog that appears, click the Environment tab and update any field(s) to their desired value(s).
Click REVIEW & RUN to apply any changes and cleanly reload Spark.