toString
is not a property, it's a function - toString()
, thus should be called as such. You're also not changing the value of the variable you call it on - you need to assign the return value to [another] variable:
$(function() {
$('#zahlungsart_0').click(function() {
var gesamtsumme_neu = Number($('#gesamtsumme').attr('rel'))+6;
var newVal = gesamtsumme_neu.toString().replace('.',',')
console.log(newVal );
$('#gesamtsumme').text(newVal);
});