Take a charecters from file replace by next charecter

#include<stdio.h>
int main()
{
        char ch;
        FILE *fs,*fd;
        fs=fopen("1.txt","r"); //File where input is present
        fd=fopen("cat2.txt","w");// file where we are writing the out put

        while(ch!=EOF)
        {
                ch=fgetc(fs);
                if(ch>=97 && ch<=121)
                {
                ch++;
                fputc(ch,fd);
                }
                else if(ch==122)
                {
                  ch='a';
                fputc(ch,fd);
                }
        }
        fclose(fs);
	fclose(fd);
}