cambiar palabras de <span>

[America|UNK]

Código Javascript:

<script type="text/javascript">
    (function(t,s,n){
        var E = document.getElementsByTagName(t),i,l
        for(i=0,l=E.length;i<l;i++){
            if(E[i].innerHTML.match(new RegExp("^"+s+"$","i"))) E[i].innerHTML = n
        }
    })("span","pagar","PAGA AHORA")
</script>
 

Al final del body