Simply push "Check for Updates" in your router,What's new in 6.6 (2013-Nov-07 13:04):
*) winbox - fixed problem where all previous session opened windows were read only;
*) certificate - no more 'reset-certificate-cache' and 'decrypt' commands,
private keys can be decrypted only on 'import', use 'decrypt' before upgrade if needed;
*) fixed arp-reply only with more than one ip address on interface;
*) fixed RB400 not to reboot by watchdog during micro-sd format;
*) web proxy - fix SPDY server push handling;
*) certificate - merged '/certificate ca issued', '/certificate scep client' and
'/certificate templates' into '/certificate';
*) console - :foreach command can iterate over keys and values in an array,
by specifying two counter variables, e.g.:
:foreach k,v in=[/system clock get] do={:put "$k is $v"};
*) added support for new Intel 10Gb ethernet cards (82599);
*) certificates - fixed certificate import;
*) wireless - fixed crash when dfs was enabled on pre-n wireless cards;
*) fixed port flapping on CCR;
or download from http://www.mikrotik.com/download