To install SSL certificate for your websites from Plesk, please follow the following steps:-
1) Login to your Plesk control panel.
2) Click on "Domains" and then Click on "Control panel" in front of the domain whose files permession you want to change.
3) Click on "Websites and Domains".
4) Click on " Show Advanced Operations ".
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 recieved the email back from your CA which contain your certificate and key then upload
them to the Certificate respositary in Plesk and click on Send text.