Your ram cpu and motherboard are both capable of running 2400mhz.
Though, this is an old platform and your ram might be 2133 too, so please check this first. (Should be written on the small sticker on the ram stick)
If it is faster than 2133 (2400, 2666, 3000 ect.) then you have a few options.
Please update your bios before proceeding with any type of bios tinkering.
Firstly, use XMP, which should be a 1 click ordeal.
Enter your bios, and then enter advanced mode (Usually F7 on asus, but it should be written somewhere on the front page.)
I believe XMP shows up on the first page, if not, enter the tuning/overclocking tab and look for XMP settings.
You may also do this manually, by going in the tuning/overclocking tab, and looking for ram speed, and just manually changing this to 2400mhz, Though I would not recommend this since overclocking ram willy nilly is pretty flaky.
You won't cause any damage to your hardware by doing this, in the worse case scenario that your PC won't boot after changing said ram speed, just reset CMOS.
If you would like to know more about manual RAM overclocking, there are tons of indepth guides on the internet.
I would also like to mention that you would not get much if any added performance from upgrading to 2400mts from 2133mts, 3-5% at most.