Can I paste these to the main file.htaccess

Yes, that was the first in your search results. Thank you

Done sir I have replaced the .htaccess file with this new one which you provided

Did you check your site? Is the 404 not found issue resolved?

Yes the issue is resolved thanks for your support.
One more question can I change the background image of login page .

Yes, you can change the images in `themes/default/admin/assets/images/login-bgs/` Thank you

I have already changed the img but the file name is same as you assigned but no changes implemented the image appears same after changing. One more question can I change warehouse term to shop term and also can I add more payment type option like paytm,gpay etc

Hello,

There are multiple images and will be displayed randomly. You can delete those you don't want.

You will need to hire developer to add payment gateways.

Thank you