how to change note value

see: https://musescore.org/en/handbook/note-input#duration
https://musescore.org/en/handbook/accidental
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Also, if you have already entered a note of one duration and wish to change it to another duration, simply click it (while *not* in note input mode) then select the desired notation (using the toolbar or keyboard shortcut). But normally, as noted above, you select the duration *before* entering a note.