Step 1: Login to CCP and chose for the webhosting plesk in the left menu 

 

Step 2: click on the tile of your website

 

Step 3: click on "more tools".

 

 

Step 4: within Plesk choose "hosting settings"Step 5: under security check the checkbox “Permanent SEO-safe 301 redirect from HTTP to HTTPS” and select the appropriate certificate in the drop down list.

 

Step 6: click on apply to safe the settings. From now HTTP will be redirected to HTTPS

Reference: https://www.plesk.com/blog/various/http-to-https-heres-what-to-do/