Properties pane of main window has Quality tab. In addition to the settings for automatic quality selection this tab also contains the way to select quality manually.
In order to do this click Analyze button to retrieve available qualities for the clip. Alternatively can be used menu item of the same name from the clip list context menu.
Selected streams combination is stored between program runs and the clip is always re-analyzed before download.
If selected combination of the streams will be not found after analysis, download will be stopped with the error "part of manual selection was not found". Under this tab in such case will be restored streams and original selection that were available before current analysis.