To redirect all HTTP traffic to HTTPS using the .htaccess file, you can add the following code:

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

This code uses mod_rewrite module in Apache to check if HTTPS is not already enabled and then redirect the request to the same URL but with the HTTPS scheme instead of HTTP.

Note: Make sure you have SSL installed on your server before implementing this code, otherwise the website may not work properly.