Use the following code the in .htacess file
ExpiresActive On
ExpiresByType image/jpg “access 1 year”
ExpiresByType image/jpeg “access 1 year”
ExpiresByType image/gif “access 1 year”
ExpiresByType image/png “access 1 year”
ExpiresByType text/css “access 1 month”
ExpiresByType application/pdf “access 1 month”
ExpiresByType application/javascript “access 1 month”
ExpiresByType application/x–javascript “access 1 month”
ExpiresByType application/x–shockwave–flash “access 1 month”
ExpiresByType image/x–icon “access 1 year”
ExpiresDefault “access 2 days”