Change HighCharts axis title

Yes you can do this by using the following:

chart.yAxis[0].axisTitle.attr({
        text: 'new title'
    });

This can be done directly on the Axis object using setTitle now. For example:

chart.yAxis[0].setTitle({ text: "Bananas" });

See this JSFiddle demonstration. The method signature is:

setTitle(Object title, [Boolean redraw])

So you could optionally pass a boolean to wait with redrawing. The title object takes the same parameters as xAxis.title meaning you could pass in styles and several other options as well as the text itself. The API documentation has the complete information.