MuseScore -- Index -- Beams

Beams are set automatically but the automatic setting can be manually overridden. For this drag a beam symbol from the beam palette to a note were you want to change the beam behaviour.
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 a this note.
Do not beam this note.
Start a second level beam at this note.

See also: Cross staff beams