In a normal emacs (not spacemacs), discard change is bound to key of "k", then in spacemacs, base on the following information.
http://spacemacs.org/doc/DOCUMENTATION.html
On the other hand, anything originally bound to k will be found on K, since k is reserved but K is not. If there is a binding on K, that will be moved to C-k.
But when I press K, it pops us a helm buffer to let me select a file to ingore in git.