To cycle through Timecode Base, or Frames / Feet + Frames (depending if you have the “Use Feet + Frames” option checked in the Project Settings), Ctrl-click (Windows) or Command-click (Mac OS) the current-time display. The current-time display is in the upper-left corner of the Timeline panel and at the bottom of the Layer, Composition, and Footage panels. (See Timeline panel.) The option that is not selected in Project Settings will be displayed as smaller text underneath.
To change time display units, choose File > Project Settings, and choose from the options in the Time Display Style section.