How do I write what is inside a character array to a text file?

if you read the file, one char at a time, for instance using fgetc() then you could check if the character is a A and if it is an A then call fseek( file, -1, SEEK_CUR ); followed by fputc( 'B', file );

The following proposed code:

1. cleanly compiles
2. performs the desired functionality
3. is not the fasted possible implementation, however, to keep the code simple, I used the slower fgetc() and fputc() rather than fread() and fwrite()
4. properly checks for errors when calling fopen() and fgetc() but skipped error checking for fseek() and fputc(). You can easily add the skipped error checking. Read the MAN pages for those functions for details

and now, the proposed code:

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

void Function( void )
{
    FILE * pFile = fopen("example.txt", "r+");
    if( ! pFile )
    {
        perror( "fopen failed" );
        exit( EXIT_FAILURE );
    }

    int ch;
    while(  (ch = fgetc( pFile )) != EOF )
    {
        if( ch == 'A' )
        {
            fseek( pFile, -1, SEEK_CUR );
            fputc( 'B', pFile );
        }

        //else
        //{
        //    fputc( pFile, ch );
        //}
    }
    fclose( pFile );
}


int main( void )
{
    Function();
    return 0;
}

Maybe my code can help you. (the explication in the code)

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

void Function()
{
    FILE* fp = fopen("input.txt", "r+"); // Open file to read and write
    if (fp == NULL) return; // exit if you can not open the file
    char ch;
    while ((ch = fgetc(fp)) != EOF) // read character by character until end of the file
    {                                             
        if (ch == 'A') // if we meet A character
        {
            fseek(fp, - 1, SEEK_CUR); // set the position of the stream one character back
            fprintf(fp, "%c", 'B'); // write B character to position of A character
        }
    }
    fclose(fp);
}

int main() {
   Funtion();
   return 0;
}

The input and output:

$cat input.txt
AAA BBB AAA BBAA

The file after replacing:

BBB BBB BBB BBBB

As mentioned above [by Andreas], you probably want r+ as the open mode.

Although you can use fread [and fwrite] with fseek as suggested, I think fgetc and fputc would be simpler.

#include <stdio.h>

int
main(void)
{
    long pos;
    int chr;

    FILE *pFile = fopen("example.txt", "r+");

    while (1) {
        chr = fgetc(pFile);
        if (chr == EOF)
            break;

        if (chr == 'A') {
            fseek(pFile,-1,SEEK_CUR);
            fputc('B',pFile);
        }
    }

    fclose(pFile);

    return 0;
}

UPDATE:

Others have suggested using fread, scanning the block, doing fseek with a rewrite of the block using fwrite.

For that, I'd just use open/read/write/lseek/close or pread/pwrite.

But, if we're going to go to that much trouble, as I mentioned in my top comments, I'd use mmap. It's still pretty simple, and it is much faster than any of the aforementioned methods.

Anyway, here's the code:

#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/mman.h>

int
main(void)
{
    int fd;
    struct stat st;
    ssize_t remlen;
    char *base;
    char *cur;
    char *end;

    fd = open("example.txt",O_RDWR);
    fstat(fd,&st);

    base = mmap(NULL,st.st_size,PROT_READ | PROT_WRITE,MAP_SHARED,fd,0);

    remlen = st.st_size;
    cur = base;
    end = &cur[remlen];

    for (;  remlen > 0;  remlen = end - cur) {
        cur = memchr(cur,'A',remlen);

        if (cur == NULL)
            break;

        *cur++ = 'B';
    }

    munmap(base,st.st_size);
    close(fd);

    return 0;
}