Open a MIDI track in Ableton's Arrangement or Session view. In the Arrangement view, double-click the MIDI track in the upper area of the interface. It opens in the Piano Roll view at the bottom of the interface.  In Session view, click anywhere in the MIDI track. Navigate to the piano roll, and highlight the note on which you want to increase or decrease the note value.  If you are adjusting all notes, press CTRL + A (Win) or CMD + A (Mac).  A stretch marker is displayed above the selected notes. This allows you to change the value of the beat of the notes you have chosen. Disable the grid temporarily by holding down the CTRL key (Win) or CMD (Mac). Hold down the SHIFT key and use the marker to increase or decrease the note values of your MIDI notes.