I was able to change the columns in the Editable Grid of sales documents by modifying the file in the path ‘apps/erpnext/erpnext/selling/sales_common.js’. The columns are defined using the following statements:
this.frm.get_field('items').grid.editable_fields = [
{fieldname: 'item_code', columns: 4},
{fieldname: 'qty', columns: 2},
{fieldname: 'rate', columns: 2},
{fieldname: 'amount', columns: 2}
];