Intercambiar los datos de la columna 1 con la ultima columna

#include <stdio.h>
#include <conio.h>

void llenar (int M[20][20], int f, int c)
{
for (int i = 1 ; i <= f ; i++)
{
for (int j = 1 ; j <= c ; j++)
{
printf ("Inserte pos:[%d][%d]: ",i,j);
scanf("%d",&M [i] [j]);
}
}
}

void mostrar (int M[20][20], int f, int c)
{
for (int i = 1 ; i <= f ; i++)
{
printf("\n");
for (int j = 1 ; j <= c ; j++)
{
printf ("[%d]",M [i] [j]);
}
}
}

void intercambiar (int M[20][20], int f, int c)
{
for (int i = 1 ; i <= f ; i++)
{
int aux = M [i] [1];
M [i] [1] = M [i] [c];
M [i] [c] = aux;
}
}

int main ()
{
int M [20] [20];
int f, c;
printf ("Inserte filas de la matris: ");
scanf("%d",&f);
printf ("Inserte columnas de la matris: ");
scanf("%d",&c);

printf ("\nLLENANDO MATRIS : \n");
llenar (M, f, c);
printf ("\nLA MATRIS ORIGINAL : ");
mostrar (M, f, c);
printf ("\n\nLA MATRICES INTERCAMBIADA : ");
intercambiar (M, f, c);
mostrar (M, f, c);
getch();
}