You can transpose whole flows or specific selections, including selected key signatures, using the Transpose dialog.
Dorico Pro automatically shows the appropriate key signatures for transposing instruments in transposing layouts.
Procedure
Result
All notes in your selection, or all notes in the flow if nothing was selected, are transposed according to the interval or number of divisions of the octave specified in the Transpose dialog. If your selection included key signatures and you activated Transpose key signatures, all key signatures in the selection are also transposed.
Any figured bass shown on the affected staves is also transposed accordingly.
Key signatures that apply to all staves are transposed on all staves in the layout, even if your selection did not include all staves. Key signatures that apply only to single staves are transposed if they are included in a selection, but this does not affect any other staff in the layout.