I want to delete a note

It sounds as though you are in Note Entry Mode and are using the mouse to enter notes. Make sure you are not currently in Note-Entry Mode (press [Esc] or deselect by clicking on the toolbar N button).

A] Left-click on note and press [Del] -> note deleted, or...
B] Left-click on note and type a,b,c,d, etc. -> note is replaced with respective note typed.

You don't need to delete a note at all to replace it - just replace it. Type the new note directly over the old, it replaces it. Deleting it is just an unnecessary extra step. But for the record, to delete a note (which replaces it with a rest) simply click it while not in note input mode and press Delete. So you are welcome to delete the note first before replacing it, but again, it is totally unnecessary.

It IS in the handbook on the Note Entry page:

To change the pitch of a single note, first press Esc to make sure that you're not in note input mode and that you have no other notes selected; then either drag the notehead up or down with the mouse, or else select it and use the ↑ (Up) or ↓ (Down) keys to change its pitch. You can also type the letter name of the note you want to change it to, and use Ctrl+↓ or Ctrl+↑ to correct the octave, if necessary (Mac: Cmd+↓ or Cmd+↑).

In general:
https://musescore.org/en/handbook/note-input#change-notes

Change duration:
https://musescore.org/en/handbook/note-input#change-duration

Change pitch:
https://musescore.org/en/handbook/note-input#change-pitch

Change rest to note and vice versa:
https://musescore.org/en/handbook/note-input#change-note-rest

Regards, and welcome aboard.