Click the key signature, then choose a new key signature in the Key Signature dialog.
Control-click the key signature.
If there is no visible key signature (key of C/A minor), Control-click between the clef and the time signature.
Choose Edit Key Signature from the shortcut menu.
Choose a value from the Key pop-up menu.
Click either the Major or Minor button, then click OK.
Click the time signature.
In the Time Signature dialog, enter a value for the number of beats, choose a value from the Note Value pop-up menu, then click OK.
Control-click the time signature.
Choose Edit Time Signature from the shortcut menu.
If the Time Signatures section of the Part box isn’t visible, click the Time Signature button in the Part box.
Do one of the following:
Drag one of the defined time signatures from the Part box to the place in the score where you want to add it.
Drag one of the undefined time signature symbols (A/B or A+B/C) to the place in the score where you want to add it, then, in the Time Signature dialog, enter the numbers for the Number of Beats and Note Value in their respective fields.
Detailed information about key and time signatures can be found in Apple Books | PDF