There are two ways to do this:
1. Set Now Time to the beginning of the piece (assuming you want to make this change for the entire piece)
Go into Piano Roll view for the track or tracks you want to change (you can select more than one track)
Find E3 in the Scale pane (the part that looks like a piano keyboard on the far left). Click it once. This selects every E3 in the track or tracks you're editing.
Then click any E3 note in the roll view (they are all selected) and drag it down to D3. All the selected notes will move to wherever you drag the note you've grabbed. There might be some lag if there are a lot of notes being moved, but they will move.
or use the function that used to be called Interpolate but is now called Process|Find/Change... This is a little more complicated but is very powerful for many tasks.
2. Drag select the area you want to change (or use Ctrl-A to select the entire piece)
Go to Process|Find/Change... the Event Filter - Search dialog will open
At the very bottom right of the dialog you'll see two buttons, All and None. Click None. This lets you start completely fresh.
At the upper left in the Include section, the first type of thing to select is Note. Check that box. Next to that, you can enter the key note/octave you want to select. Leave "exc" unchecked; in Min, enter C 3 and in Max also enter C 3 then click OK. This will select
only the note C3 in the range of time/tracks you initially set for changing.
When you click OK, the Event Filter - Replace dialog opens. It's the same dialog, but now you set what you want your note selection to be changed to. In Key Min, enter D 3; in Max, enter D3. Click OK.
The dialog will close and now every C3 in the range you selected will have been changed to D3. You see how this function can be used to do much more extensive search/replace and other transformations. If you need more information, read about Event Filters in the Sonar manual or in-program help.
'Hope that solves your issue.
mag
Edit: Looks like Arachnaut found another way, which you also could have done with Process|Transpose... Lots of ways to do things in Sonar.