Normally, when the AirPort Express is configured to "Join" or "Extend" a wireless network, it will look at both the 2.4 GHz band and 5 GHz band and choose the best quality, or strongest signal......which is almost always going to be 2.4 GHz if the signal has to pass through any obstructions like walls or ceilings.
You can "force" the Express to connect to the 5 GHz band, but in order to do so, you must setup the Time Capsule to provide a separate name for the 5 GHz network. Once that is done, you can "point" the AirPort Express to that specific network.
But....there is no guarantee that connecting to 5 GHz will provide you with a faster connection. If the signal must pass through several obstructions to reach the Express, the 5 GHz signal may well be slower than the 2.4 GHz signal at that point.
Post back if you need more details about how to set this up. But, also keep in mind that when you give the Express the ability to choose the best quality signal....as it is doing now....it is connecting to 2.4 GHz because that is the better quality signal....and likely faster signal.