Transpose dialog
Using the transpose dialog, it is easy to transpose the entire score to another key.
You'll arrive at the transpose dialog using 
, <ctrl-F3> or [Part|Transpose].
The dialog uses the circle of fifths to show the possible keys along with the number of sharps or flats corresponding to the key.
Select the desired key using arrow-keys or the mouse. If you were transposing to obtain lower written notes and get messages like 5 semitones up you need to use the octave shift button to change that to 7 semitones down which is probably more of your liking.
By default all keys from 4 flats to 7 sharps are displayed. Use the [7 flats] checkbox to switch to the 7 flats-4 sharps range. Use the Minor checkbox to select minor clefs instead of major keys.
Even text files containing chords may be transposed this way.