Click on the "Selection Tool" in the "Main Tool Palette." This tool looks like an arrow inside of a box with a broken dotted line. It is the first tool in the palette under the default settings. It will be in a different place if you have changed the order of the tools.
Select the staff at the top of the score by clicking on it once. The measure will turn blue to indicate that it has been selected.
Right-click on the measure to be changed. A drop-down menu will appear. Select the "Clef..." option from the menu.
Enter the parameters for the clef change in the dialog box that appears. Select the clef from the list of available clefs. There are typically 16 different clefs to choose from depending on the version of Finale. Enter in the options for the measures you would like to clef to begin and end. Press "OK."