issue in changing contents of file from lower case to upper case

Question

I am trying to change the contents in a file from lower to upper case by using the code below. It got stuck in an infinite loop. What is the mistake here?

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

int main()
{
    FILE *fp = fopen("file.txt", "r+");
    if (fp == NULL)
    {
        printf("error occured");
        exit(1);
    }
    char c;

    while ((c = fgetc(fp)) != EOF)
    {
        if (islower(c))
        {
            fseek(fp, -1, SEEK_CUR);
            fputc(toupper(c), fp);
        }
        //fseek(fp, 0, SEEK_CUR);
    }
    fclose(fp);

    return 0;
}

The programme is working fine if the commented line is uncommented.

If possible, please mention some good resource for knowing about fseek function in detail.