Some of the elements of the presentation seem to be uneditable. (for example the footer or the header of the title slide)
What should I do to modify these?
These elements are not editable in the normal view because they are in the Slide Master view. To edit the Slide master, just go to View>Slide Master and you will see the Slide Master and the associated layouts.
For more information about the Slide Master, got to the dedicated page on Microsoft Office Suport.