How to use strcat in c?

Hi,
Today I decided to code a small program that could make a backup file.
I wanted to change the extension of the given file to "BAK" to be able
to make a backup file.

So I coded the below program but when I tried to use strcat(); function
to change the extension of the given file to a backup file I faced a problem
and I was not able to handle it.

Finally after pondering a lot,I changed my program and used a for loop
to be able to solve the problem.
You can see this loop in the second program which is below.
But it was not what i wanted to do.
I wanted to use strcat(); function and I hope someone can help me
to correct the first program.

Thanks in advance.
Best Regards,
Zest.

The imcomplete program:

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

#define SIZE 40
int main(int argc,char * argv[])
{
	FILE *in,*out;
	char name[SIZE],chr = '.',bk[] = "Bak",ch;
	char *occurance;


	if(argc != 2)
	{
		puts("This program makes a BackUp of your files");
		fprintf(stderr,"Usage: %s filename\n",argv[0]);
		exit(EXIT_FAILURE);
		}
		if((in = fopen(argv[1],"r")) == NULL)
		{
			fprintf(stderr,"I can't open the file %s",argv[1]);
			exit(EXIT_FAILURE);
			}


			strncpy(name,argv[1],SIZE-5);
			name[SIZE-5] = '\0';

			occurance = strchr(name,chr);

			strcat(occurance , bk);

			if((out = fopen(name, "w")) == NULL)
			{
				fprintf(stderr,"I can't copy to the file %s",name);
				exit(EXIT_FAILURE);
				}

				//Copying Data

				while((ch = getc(in))!= EOF)
				putc(ch,out);
				
				if(fclose(in) != 0 || fclose(out) != 0)
				fprintf(stderr,"Error is closing files.\n");

				puts("Done!\n");



				getch();
				return 0;

				}

The program with for loop:

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

#define SIZE 40
int main(int argc, char* argv[])
{	
	FILE *in,*out;
	char name[SIZE],chr = '.',bk[] = ".Bak",ch;
	char *occurance;


	if(argc != 2)
	{
		puts("This program makes a BackUp of your files");
		fprintf(stderr,"Usage: %s filename\n",argv[0]);
		exit(EXIT_FAILURE);
		}
		if((in = fopen(argv[1],"r")) == NULL)
		{
			fprintf(stderr,"I can't open the file %s",argv[1]);
			exit(EXIT_FAILURE);
			}


			strncpy(name,argv[1],SIZE-5);
			name[SIZE-5] = '\0';

			occurance = strchr(name,chr);
			
			for(int i = 0; i < 4; i++)
			   *(occurance + i) = bk[i];

			

			if((out = fopen(name, "w")) == NULL)
			{
				fprintf(stderr,"I can't copy to the file %s",name);
				exit(EXIT_FAILURE);
				}

				//Copying Data
				
				while((ch = getc(in))!= EOF)
				putc(ch,out);

				if(fclose(in) != 0 || fclose(out) != 0)
				fprintf(stderr,"Error is closing files.\n");

				puts("Done!\n");



				getch();
				return 0;

				}