check an uncheck tick box

Hi hopefully this is a simeple one. I have an 'turn on' and ' turn off' checkbox

I have found a pice of java attached to 'tun off' checkbox 'on mouse up' action. this unchecks the three other children checkboxes. it works fine.

how do i do the opposite.

i just want to add the opposite script to the mouse up action of a different box. to add the tick back in again. check all the same boxes. so that they all turn back on agian.  

if (event.target.value != "Off") {

this.getField("chkSGD").value = "Off" ;

this.getField("chkInternet").value = "Off" ;

this.getField("chkEmail").value = "Off" ;

}

thanks