<p>In the C language, using keyboard interrupt, how can I display an alternate key from what the user typed? E.g., when I press 'Q' on the keyboard, then the screen should display 'A'.</p>

We need a little more information about Your environment.
As far as I understand Your question, You want to replace the
keyboard interrupt handler with Your own in this way:
Save the entry address of the original keyboard handler.
Install Your own that calls the original and manipulates the keycode value.
However, on most modern operating systems it is much easier to define and install a new keyboard layout.