Are you asking how to setup an AP Extreme AC as an access point to extend the network? You probably don't want to do that, the AP line has a single radio that is shared between uplink and client connections, needs to be in close proximity to the main router (for signal strength). Far better performance using Ethernet to connect it to the main router, even if you need to use power line or MOCA adapters to extend WiFi to the secondary location.
That said, in the settings for the unit, set wireless network mode to Extend a Wireless Network. But, I seem to recall Extreme doesn't support this setting, only Express... I may be wrong. If you don't see the Extend option, it is because it is not supported.