How to double the height and width of a ppm file

Question

I'm trying to build a really basic PPM picture editor. I've got it to where I can open the file, and print out a copy of the picture in a different file. Now I'm trying to work on growing/shrinking the picture. Right now, I'm trying to figure out how to double the size of a PPM file so that the picture is 2x bigger than before.It's all on the command line because it's not very sophisticated and I'm pretty new at programming. Does anyone have an idea on how I can do this? Here is the code I'm using to read/print the file (from someone else on here):

#include<stdio.h>
#include<stdlib.h>

typedef struct {
     unsigned char red,green,blue;
} PPMPixel;

typedef struct {
     int x, y;
     PPMPixel *data;
} PPMImage;

#define CREATOR "RPFELGUEIRAS"
#define RGB_COMPONENT_COLOR 255

static PPMImage *readPPM(const char *filename)
{
         char buff[16];
         PPMImage *img;
         FILE *fp;
         int c, rgb_comp_color;
         //open PPM file for reading
         fp = fopen(filename, "rb");
         if (!fp) {
              fprintf(stderr, "Unable to open file '%s'\n", filename);
              exit(1);
         }

         //read image format
         if (!fgets(buff, sizeof(buff), fp)) {
              perror(filename);
              exit(1);
         }

    //check the image format
    if (buff[0] != 'P' || buff[1] != '6') {
         fprintf(stderr, "Invalid image format (must be 'P6')\n");
         exit(1);
    }

    //alloc memory form image
    img = (PPMImage *)malloc(sizeof(PPMImage));
    if (!img) {
         fprintf(stderr, "Unable to allocate memory\n");
         exit(1);
    }

    //check for comments
    c = getc(fp);
    while (c == '#') {
    while (getc(fp) != '\n') ;
         c = getc(fp);
    }

    ungetc(c, fp);
    //read image size information
    if (fscanf(fp, "%d %d", &img->x, &img->y) != 2) {
         fprintf(stderr, "Invalid image size (error loading '%s')\n", filename);
         exit(1);
    }

    //read rgb component
    if (fscanf(fp, "%d", &rgb_comp_color) != 1) {
         fprintf(stderr, "Invalid rgb component (error loading '%s')\n", filename);
         exit(1);
    }

    //check rgb component depth
    if (rgb_comp_color!= RGB_COMPONENT_COLOR) {
         fprintf(stderr, "'%s' does not have 8-bits components\n", filename);
         exit(1);
    }

    while (fgetc(fp) != '\n') ;
    //memory allocation for pixel data
    img->data = (PPMPixel*)malloc(img->x * img->y * sizeof(PPMPixel));

    if (!img) {
         fprintf(stderr, "Unable to allocate memory\n");
         exit(1);
    }

    //read pixel data from file
    if (fread(img->data, 3 * img->x, img->y, fp) != img->y) {
         fprintf(stderr, "Error loading image '%s'\n", filename);
         exit(1);
    }

    fclose(fp);
    return img;
}
void writePPM(const char *filename, PPMImage *img)
{
    FILE *fp;
    //open file for output
    fp = fopen(filename, "wb");
    if (!fp) {
         fprintf(stderr, "Unable to open file '%s'\n", filename);
         exit(1);
    }

    //write the header file
    //image format
    fprintf(fp, "P6\n");

    //comments
    fprintf(fp, "# Created by %s\n",CREATOR);

    //image size
    fprintf(fp, "%d %d\n",img->x,img->y);

    // rgb component depth
    fprintf(fp, "%d\n",RGB_COMPONENT_COLOR);

    // pixel data
    fwrite(img->data, 3 * img->x, img->y, fp);
    fclose(fp);
}

void changeColorPPM(PPMImage *img)
{
    int i;
    if(img){

         for(i=0;i<img->x*img->y;i++){
              img->data[i].red=RGB_COMPONENT_COLOR-img->data[i].red;
              img->data[i].green=RGB_COMPONENT_COLOR-img->data[i].green;
              img->data[i].blue=RGB_COMPONENT_COLOR-img->data[i].blue;
         }
    }
}

int main(){
    PPMImage *image;
    image = readPPM("can_bottom.ppm");
    changeColorPPM(image);
    writePPM("can_bottom2.ppm",image);
    printf("Press any key...");
    getchar();
}