You turn the best one in.
In fact, if you want to send more than one score at schools that mix and match (a bud is applying to Stanford, and it seems from the app they mix), you have to pay once for each report you're sending.
I took it three times. I knew I messed up the first time when I ran out of time on the reading (I scored a 32). The second time, my Science Reasoning came back low, and I knew I messed up (composite was a 32, though a higher 32 this time). Third time I took it, I only messed up a little on Math and Science, but I totally lucked out on my Science. Composite is a 34 now!
Moral: take again only if you can clearly identify where you've messed up on the previous test(s).