MuseScore -- Index -- Edit Mode

Many elements in the score can be edited in Edit Mode :

Double Click    start Edit Mode
Escape end Edit Mode

Some elements show handles in edit mode which can be moved by mouse dragging or keyboard commands.

Slur in edit mode :

Sample slur in edit mode

Available keyboard commands:

Left    move handle left one Space
Right move handle right one Space
Up move handle up one Space
Down move handle down one Space
Ctrl + Left move handle left 0.1 Space
Ctrl + Right move handle right 0.1 Space
Ctrl + Up move handle up 0.1 Space
Ctrl + Down move handle down 0.1 Space
Tab go to next handle

See also Text Editing, Slurs, Brackets, Lines.