How to change note value in Ableton Live
When you draw in a note in Live's MIDI Clip View editor, the note defaults to the current size of the grid. You can change the grid size (there's a right-click menu, as well as keyboard shortcuts, and zooming in/out if that is enabled), and you can also change the length of an existing note by grabbing the beginning or ending edge of it and dragging around in whatever the current grid size is. You can also drag to arbitrary sizes, ignoring the grid, by holding ctrl/cmd.

If you select a group of notes and then change one's length, they'll all expand/contract with it.