Sony a7S III Instruction Manual page 12

To play back movies
Press the (Playback) button to switch to the playback mode. Select a
movie to play back using the control wheel, and press the center of the
control wheel to start playback.
To change the exposure mode for movies (P/A/S/M)
MENU  (Shooting)
the desired exposure mode depending on the subject or purpose of
shooting (page 16).
GB
12
 [Shooting Mode]  [Exposure Mode]  Select