Open up the Zeppelin UI, then click on "Interpreter" within the dropdown menu in the upper right-hand corner.
From there, you can scroll down to the Spark interpreter (or do a search for "python") and you will see a field called "zeppelin.pyspark.python".
You can change this value to point to your alternative python location (i.e. change python to something like /path/to/new/bin/python).
Let me know if that helps.