To install SSL certificate for your websites from Plesk, please follow the following steps:-
1) Login to your Plesk control panel.
2) Click on "Subscriptions" and then Click on "Manage Hosting" in front of the domain whose files permission you want to change.
3) Click on "Websites and Domains".
4) Click on " Secure Your Sites ".
5) Click on "ADD SSL Certificate".
6) Enter the required details to create the SSL Certificate and click on "Request".
7) Click on the Certificate that you have created.
8) Copy the CSR certificate and Private key and Email both of them to your CA (certification authority).
9) Once you have received the email back from your CA which contain your certificate and key then upload
them to the Certificate repository in Plesk and click on Send text.
