How can I rename the “wp-admin” URL without any problem ??
You can’t.
*Drinks more coffee*
Wow, good coffee. ??
If you try and renamed that wp-admin
directory you will break your installation in terrifying ways that are too horrible to contemplate. Really, it is a Very, Very Bad Idea? it’s not supported and see “terrifying” comment above.
On the Bad Idea? scale of 1 to 10 it’s about a 1 billion.
*Rereads text just written and thinks about switching to decaf*
If you want to harden your installation then there are ways to safely do that. See this article.
https://codex.www.ads-software.com/Hardening_WordPress
There are also security plugins too.
https://www.ads-software.com/plugins/search.php?q=security
Except for Jetpack’s Brute Protect I myself don’t use any of them. I only use Jetpack’s option because I’m a fan of data collection projects. ??
https://jetpack.me/2015/03/17/jetpack-3-4-protect-secure-and-simplify/