When you create a new project, by default it has one key signature that lasts for the entire project.
When Show Advanced Tools is selected in the Advanced preferences pane, you can add key signature changes to a project in the Signature List, Signature track, or Score Editor.
Do one of the following:
Click the word Time in the Signature List and choose Key from the pop-up menu, then click the Add Signature button 
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Click a position in the Signature track’s key signature area with the Pencil tool, then choose a key in the Key Signature dialog.
You may need to expand the Signature track until you see separate areas for time and key signature changes.
In the Key Signature dialog, you can enable the use of double flats and sharps by deselecting the “Disable bb, x” checkbox.
In the Score Editor, double-click a staff between the clef and time signature, then choose a key in the Key Signature dialog. You can also drag the key signature from the Part box to the target time position.