Beams are set automatically, but the automatic setting can be overridden manually. Drag a beam symbol from the beam palette to a note in order to change its beam behavior.
You can also first select a note, and then double-click the appropriate symbol in the beam palette.
Start a beam at this note.
Do not end a beam at this note.
Do not beam this note.
'A' = (back to) Automatic mode: the mode MuseScore chooses on note entry, dependent on current time signature.
To change the beam angle or the distance of the beam to the notes (i.e. the length of the stems), double click on the beam to put it into Edit mode, with the right end handle being selected. Up/down arrow will now change the angle. Selecting left end handle and using the up/down arrow keys will lengthen/shorten the stems. Hit Esc to get out of edit mode, once done with the changes.
To move a beam from above to below the notes, or vice-versa, flip the direction of the stems by using the button along the second top row (before the voice indicators) that shows a note with stems attached above and below or use the X key.