Upgrading PostgreSQL 9.3 to 9.5
Once you drop cluster for version 9.5, you may upgrade your 9.3 to 9.5 with this command:

sudo pg_upgradecluster -v 9.5 9.3 main

If you omit -v 9.5 it will upgrade the cluster to the latest postgresql version you have installed. For more information, type man pg_upgradecluster.

Now, since you already upgraded to 10, you have to stop the new cluster (version 10) and start the 9.3 again. You may do this with commands:

sudo pg_ctlcluster 10 main stop
sudo pg_ctlcluster 9.3 main start