@Cris2 you need to open developer tools in your browser and use inspect element to highlight necessary label that you would like to change (see example below):

Then you can use for property and write something like this to change the label:
$(document).ready(function() {
$("label[for='Subgrid_1']").text("New test label");
});
You can add this code into Custom Javascript field under Additional Settings tab in your basic form.