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 editmode which can be moved by mouse dragging or keyboard commands.

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> goto next handle

See also Text Editing, Slurs, Brackets, Lines.