note beams

Select the stem or beam of a note, and look for the Direction dropdown in the Inspector (F8 to show if it's hidden). Those can be a bit hard to click, so you can also click the notehead, and click the Stem or Beam buttons at the bottom of the inspector.
Or if you're trying out the MuseScore 4 beta, click any part of the note, find the Stem tab under the Note section in Properties, and click the up or down arrow.

Select the beam, press X