Open your .htaccess file and update as following
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}/$1 [R=301,L]
RewriteCond $1 !^(index\.php|images|robots\.txt|assets)
RewriteCond %{REQUEST_URI} !/system/.* [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php/$1 [L]
</IfModule>
