function islem(ne)
{
    var deger = parseInt($('#sayimiz').html());
    if(ne=='+')
    {
         deger = deger+1;
    }
    else if(ne=='-')
    {
       deger = deger-1;
    }
    else
   {
       alert('HATA');
   }
   $('#sayimiz').html(deger);
}

<span id="sayimiz">5</span>
<a href="javascript:islem('+');">Arttır</a> | <a href="javascript:islem('-');">Eksilt</a>