Save the old Spark configuration and add symlink from /etc/spark/conf:

mv /etc/spark/conf /etc/spark/conf.saved

ln -s /usr/hdp/current/spark-client/conf /etc/spark/conf

ls -la /etc/spark

total 4 
drwxr-xr-x 3 root root 4096 Jun 19 21:51 2.3.0.0-2323 
lrwxrwxrwx 1 root root 35 Jun 19 21:54 conf -> /usr/hdp/current/spark-client/conf 
drwxr-xr-x 2 root root 4096 Jun 14 00:11 conf.saved

From Ambari Web, browse to Services > Spark.

From the Service Actions menu, select Start.

After Spark has started, select Run Service Check. Confirm the Service Check passes.