To do this, go to the Image menu and choose Mode > Indexed Color.
The indexed color dialog box appears. You can read more about the options in the dialog in the online help, but for this exercise, just make the following selections:
Palette: Local (Perceptual)
(To learn more about these options, see the topic "Conversion options for indexed-color images" in Photoshop Help.)
Click OK, and the image is converted to indexed color mode, although it probably won't look any different to you.