Do you mean schematic or PCB?
For PCB use the filter pane, filter on the top (or bottom) overlay layer and then text. All text on that layer will show in the highlighted objects list. You can then select individual or all and use PCB inspector to update. Or you can use CTRL-drag select to select the designators by mouse drag window. You have to use the CTRL key with mouse because the overlay text is part of a component and not independent text.
For schematic; I don't think there is a SCH Filter in CS like there is/was in AD. But something that sort of works is this. (Save your work before doing this.)
Zoom out to see the sheet.
Use home>select all (CTRL-A)
Use the SCH Inspector window
Change the include only to display only parts
Click on the Component Designator specific object. It should print up another sub-selection menu. But you might have to follow "click here to to display all objects"
Change the FONT setting for the selected items.