Key signature

Select the first measure, then click on Dmaj (two sharps) in the "Key Signatures" palette.

However, this does not change the pitch. Instead, you have to go to "Tools -> Transpose".

Here's one way:

Add the D major key signature (2 sharps) at the beginning, which replaces the E major (4 sharps). You'll now see sharp signs on all G's and D's.

Right-click on a sharped D. From the context menu, click Select / More... / Same note name (D#). This will select all D sharps. Press the Down arrow key once to change them all to D naturals. Repeat this process on the sharped Gs.

And here's another:

Add the D major key signature (2 sharps) at the beginning, which replaces the E major (4 sharps). Again, you'll see sharp signs on all G's and D's.

Press Ctrl+A (Select All). Press Alt+Shift+Up, then Alt+Shift+Down, This moves the notes to the next diatonic note above, then below the current note. So, D# moves Up to E natural, then down to D natural. (Since D# is not a diatonic tone in the D major scale.)

You can test either of these in a quick test score. Add an E key signature. Type D, E, F, ... B, C. (with a key signature of E, a D Locrian scale). Try one method, then the other and you'll see that they work correctly.

One possible, minor problem: if you had any notes that should be D# or G#, they will be changed to D or G natural.