just like when setting chord signature keys in Table Mode, the up and down buttons are used to change the signature key.  See Chord Signature Key.

In Melody mode the signature key has a direct impact on which keyboard key is selected when a melody note is selected on the staff.  So it is important to set it correctly before entering melody notes.

The first time a melody note is added to a song, the program will ask if the key signature is correct to give you the opportunity to change it.   Setting the proper key signature is important and is often initially forgotten when working with melody lines.

The signature key set in Melody Mode will also become the default signature key of new chords in the chord table.