This macro sets the text of the list box indexed item to buffer.

#define INCL_WINWINDOWMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND    hwndLbox;  /*  List box handle. */
LONG    index;     /*  Index of the list box item. */
PSZ     psz;       /*  Pointer to a null terminated string. */
BOOL    rc;        /*  Success indicator. */

rc = WinSetLboxItemText(hwndLbox, index, psz);