I have a dint in the back of my iPhone 5s can it be repaired

Your 5s is under warranty. If the Genius Bar determines it to be a factory defect, they will replace it free. If it is damage you caused, they will do an out-of-warranty replacement for $279 (...in the US. May be a different price in UK). Be sure to back it up before you go in, as you won't see it again.