Adjusting QGIS Print Composer output resolution?

I am using QGIS 2.12.

I would like to adjust the Print Composer's output resolution for "Export as Image" and "Export as PDF".

The default is 300 dpi, but I would like 600 or 1200.

Multiple websites say that it can be done, but they don't say how.

Answer

This parameter is directly available in QGIS Composer window:

EDIT : If Composition tab is not visible, you can easily set it up by right-clicking bellow the tabs bar and selecting Composition: