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 a this note.
Do not beam this note.
Start a second level beam at this note.

See also: Cross staff beaming