reduce two functions to one, simplify 2 functions into one with javsacript

Hi guys, any idea how to put these 2 functions together and call them separately? Thank you

//Function box + 
function boxMore(box,increment){
var numBox=casella.substring(1,3);
var letterBox=casella.substring(0,1);
var numnewBox=parseInt(numBox)+increment;
return letterBox+numnewBox;
}

//Function box -
function boxLess(box,increment){
var numBox=box.substring(1,3);
var letterBox=box.substring(0,1);
var numnewBox=parseInt(numBox)-increment;
return letterBox+numnewBox;
}