a triad where one note is moving and the others stay...

I am trying to make new copies of old scores i have, and one issue i have is where i have a third written in half notes - but the top line moves (so half note, quarter, quarter), and the bottom of the interval stays (so, dotted half, quarter). If i try to change the bottom half note to a dotted half, it automatically changes the top note as well. Is there a way around this? I am just starting out with this software and it seems very intuitive and i have a lot of old music that needs fresh copies (photocopies of photocopies of photocopies of liturgical chant) that i would love to make fresh copies that are readable for my choir.