Download the new RouterOS 6.4 release over the torrent network: http://www.mikrotik.com/download/routeros-ALL-6.4.torrent
Otherwise please visit http://www.mikrotik.com/download to pick exact version!

6.4 changelog:

*) wireless - improved 802.11n wireless retransmission (doesn't effect nstreme/nv2)
*) ovpn - allow to specify server via dns name;
*) winbox - fixed problem where ipv6 routes with non local link address gateway could not be added;
*) fixed watchdog on mipsle boards;
*) traceroute - added count & max-hops parameters;
*) traceroute - added back use-dns parameter;
*) fixed usb Yota LTE modem hangup;
*) console - make newly added item names always immediately available;
*) graphing - make sure that interface graphs gets preserved across reboots;