iwconfig [interface] [options]

iwconfig [interface]
    interface essid {NNN|any|on|off}
    interface mode {managed|ad-hoc|master|...}
    interface freq N.NNN[k|M|G]
    interface channel N
    interface bit {N[k|M|G]|auto|fixed}
    interface rate {N[k|M|G]|auto|fixed}
    interface enc {NNNN-NNNN|off}
    interface key {NNNN-NNNN|off}
    interface power {period N|timeout N|saving N|off}
    interface nickname NNN
    interface nwid {NN|on|off}
    interface ap {N|off|auto}
    interface txpower {NmW|NdBm|off|auto}
    interface sens N
    interface retry {limit N|lifetime N}
    interface rts {N|auto|fixed|off}
    interface frag {N|auto|fixed|off}
    interface modulation {11g|11a|CCK|OFDMg|...}
    interface commit 
Check man pages for more details.

sudo iwconfig [INTERFACE] essid [(HIDDEN)SSID]