The keystroke sequence is:
(press and hold) ESCAPE-COMMAND-CONTROL
(press and release) RESET
(when 'Normal' appears, release) ESCAPE
(release) COMMAND-CONTROL
However, the early manuals do not document the method of returning to 4 MHz.
The keystroke sequence is:
(press and hold) COMMAND-CONTROL
(press and release) RESET
(release) COMMAND-CONTROL
Both of the above sequences do a warm boot of the computer. The difference is that the ESCAPE key in the first sequence tells the Apple IIc Plus to use the 1 MHz (Normal) speed.
Here are additional techniques for changing the speed when you start the computer:
Entering the 1 MHz mode at Startup
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Powering on with the Escape key held down places the Apple IIc Plus in Normal mode (1 MHz). The word "Normal" appears in the middle of the second line of a screen full of reverse "@" and checkerboard cursors. Pay close attention to the screen: it clears quickly and the "Apple IIc +" screen appears.
To enter the 4 MHz mode at Startup
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Powering on with no keys pressed places the Apple IIc Plus in the faster mode.
One anomaly can occur during the keystroke process. In Applesoft, the Escape key toggles between cursor movement mode and text entry mode. The use of the Escape key in the above key sequences can have the side effect of toggling the cursor movement/text entry modes. If the cursor has the "+" sign in the center, you are in cursor movement mode. To return to text entry mode, press the Escape key (actually, any key) once.