SATA Express is the replacement for SATA 3.0 (which is limited to 6GB/s), and has very different connector. A single SATA Express connector is made up of two SATA 3.0 connectors and one additional connector. The two SATA 3.0 ports that make up a single SATA Express port can still be used separately, as normal, if you're not planning to plug in a SATA Express peripheral.
Your 840 EVO is a SATA 3.0 device, so will not gain any benefit from the SATA Express port (it lacks the connector to plug into it for one thing). You can still plug it into any of the SATA 3.0 ports on the motherboard, including those that form part of the SATA Express port.