accompaniment chords

Hi . When writing 3or 4 note piano accompaniment chords I sometimes like to make one or two move onto a new note while holding the other notes for another beat . how can I write the chord making the 3 a minim and one a crotchet. If I change the value of one note in the chord, the rest change with it. This is also a problem if I wish to slur the note a semitone, I have purchased "Mastering musescore" in the hope of helping with this but cannot find the action to take. Perhaps you could point to the section for this solution. I'm new to music writing tell me in easy terms. Thanks Terry