There is a misunderstanding here. Also some time is required for us to get up to speed with documentation.
If you have a 3CX FQDN, the renew process is done automatically - Yes Zero Admin. All we need is a working internet connection and the certificate will be renewed.
If you have your own certificate then you do the same thing you used to do before. If you have an IIS, you had to give the new certificate manually right? If you used an APACHE webserver, you have to put it manually in the folder. And now you have an NGINX webserver and this did not change - you need to go and put your new certs in the nginx folder.
We felt the need to delay documentation on this because if you are an admin, and want to import certificates, then I assume that whoever is doing this, knows what he is doing. So he/she should understand SSL and what you need to put in a webserver to do this.
Go here
C:\Program Files\3CX Phone System\Bin\nginx\conf\instance1
and the files should be pretty self explanatory.
If you want a new fresh cert, then obviously you changed the core of your system - your FQDN. So point your mouse in the direction of control panel for an uninstall yes.