How to change grid column header name in extjs

Suppose I have below grid. I want to change header 2 to  "Inst" from "Instance".

    var grid = new Ext.grid.GridPanel({
        store: store,
        columns: [new Ext.grid.RowNumberer({width: 30}),
            {header: "Date", width: 50, dataIndex: 'timestamp', sortable: true},
            {header: "Instance", width: 120, dataIndex: 'olt', sortable: true},
            {header: "DL", width: 50, dataIndex: 'dl', sortable: true}
        ], ...

In extjs 3.x :

   Use the code like this
  grid.getColumnModel().setColumnHeader(2,'Inst');

In extjs 4.x :

   Use the code like this.
   grid.columns[1].setText('Inst');