Hi,
When you added the extra note, it does get added to the score, rather than replacing anything (just think how confusing
that could get, if when you put in a new note, it replaced what is already there!)
The easiest way to get what you want (assuming I understand the end result you're aiming for) is:
1. Click on the dotted half note to select it, then click on the "No ." (no "dot") button in the Notes tool palette. This removes the dot from the half note.
2. Now you also have a third option, in addition to the two that Jellyall suggested, for changing the quarter note to a half note and positioning it correctly
You can click on the quarter note to select it, then hold down the "A" key on your keyboard (for "A"ttack), and press the Left Arrow key to move the attack of the note (when it first sounds) to the left. The effect is to increase the duration of the note while moving it's starting point (to the left), because the end of the note remains anchored. Press the left arrow key til the note gets "stretched out" enough to be a half note.*
Have fun
Sherry
*Similarly, if you have a note that "attacks" (begins) in the right place, but you want to change it's duration, you can select it and hold the "D" key (for "D"uration), and press either the Left or Right arrow key to shorten (L) or increase (R) the duration (length) of the note while keeping the attack (front end) anchored.