Calendar sync between to ownclouds

I use on my NAS owncloud 8.1.0 and the other have 8.2.5 and is on an external server.
I want to sync my calender and contacts from the NAS cloud to the cloud on the external server.
How I do it ?

Federated sharing of calendar and contacts is not supported yet:

btw. your owncloud setups are a bit outdated: