It depends on the version of Android it;s running.  4.3 and 4.4.2 with a kernel date of June 2, 2014 or earlier is easy.  (Kingo for the first, Towel Root for the second.)  For 4.4.4, you flash a 4.3 kernel, root, then flash the 4.4.4 kernel back (and void Knox).  For 5?  Check XDA.