In Joomla we are using usernames and passwords. It is crucial that you create a strong username and strong password to protect your Joomla website when creating a user account in your site.

Any Joomla web site that employs usernames and passwords must be administered with dedicated attention to ensuring that good security practices are followed by all users. If you as site administrator or your users are careless about how they choose usernames and passwords or store credentials, then a "hacker" or a "botnet"" may find it relatively easy to break your site's security.

Its very simple, you cant.
There is no option to change the administrator url
Here is why

On most Apache servers, PHP runs as an Apache module. This is the default method of installation. Many hosts have this setup because it is default and potentially they do not realize that it is also possible to configure PHP as a CGI. Running PHP as a CGI can be more secure whilst also avoiding file and directory ownership issues.

Attacks on a website can take many forms, but the most common form of attack through a vulnerable extension is like this

{jb_redbox}
Host: xxx4377.ovh.net
/phpgedview/individual.php?pid=I7&ged=family.ged%20%20//phpGedView/help_text_var.php?cmd=dir&PG
Http Code: 404     Date: May 04 12:20:26     Http Version: HTTP/1.1     Size in Bytes: -
Referer: -
Agent: Mozilla/5.0 {/jb_redbox}

or

One common problem with remote hosts is that they sometimes move sites to different folders on the host server. In general, this does not cause obvious problems in Joomla!. However, when you install an extension, you need to be able to write to the "tmp" directory.