Code

 Header...
 int main(){
»  int m, n;
»  float * *p=NULL;// This will become the array of pointers

»  printf("How many vectors?\n");
»  scanf("%d", &m);

»  p=xmalloc(m*sizeof *p);// Allocate the array of pointers
  // The rest of the code is now the same with 
  // the variable 'm' instead of the constant 'M'

»   printf("Size of each vector?\n");
»  scanf("%d", &n);// Find the row size

» for ( int i=0;i<m; ++i) 
    pi=xmalloc(n*sizeof *p[i]);// Allocate rows

»  p[0]1=3.14159;
  // Use p for something 

»  printf("New size of each vector?\n");
»  scanf("%d", &n);// Find new the row size

»  for ( int i=0;i<m; ++i) 
    pi=realloc(pi, n*sizeof *p[i]);// Resize the rows

  // Do something else

»  return 0;
}
 ...