Download the new RouterOS 6.6 release over the torrent network: http://www.mikrotik.com/download/routeros-ALL-6.6.torrent
Otherwise please visit http://www.mikrotik.com/download to pick exact version!

6.6 changelog:

*) 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;