how to

That's assuming the notes are already entered on the same stem - that is, as one chord. That will turn a single chord into a quaver. If you've already enteed the nites one by one with the wrong time values, I'm afriad you have to just re-type them correctly. Please see the section on Note Entry in the Handbook and/or watch the tutorial videos for more on how to enter notes in MuseScore, but to enter quavers (called "eighth notes" in much of the world), you hit 4 before typing the notes. I,m guessing you tried entering the ntes first then wanted to chane the rhythm, but it doesn't work that way.