Redirect abc.example.com to example.com/abc

I have a domain example.com with the following DNS records:

Hostname           Type     Value
example.com        A        93.184.216.34
help.example.com   A        93.184.216.34
www.example.com    A        93.184.216.34

The file /etc/httpd/sites-available/example.com.conf` is:

<VirtualHost *:80>
    ServerAdmin admin@mydomain.com
    ServerName example.com
    ServerAlias www.example.com help.example.com
    DocumentRoot /var/www/html/example.com/_site
</VirtualHost>

When accessing help.example.com or www.example.com in the browser, they both get redirected to example.com. I'd like help.example.com to be redirected to example.com/help while still keeping help.example.com in the browser address bar. How to make such redirection?