To allow browser caching you need to edit your HTTP headers to set expiry dates on certain types of files. Get .htaccess file in the root of domain, this file is a secret file but should come in FTP clients like FileZilla. You be able to edit the htaccess file with notepad or any type of text editor.
Depending on websites, files can set dissimilar expiry dates. If certain type of files is updated more commonly, you would set an earlier expiry date. When you are completed save the file as it was and not as any other extensions.