The process of adding SSL to a web site requires a few steps.

Contact your web host and ask for their help as they will have their own method of adding SSL to a web site.

This is what I usually do with my clients or my own WordPress web sites.
– Check in with the web host in regards to the process to setup SSL.
– Login to the WordPress site and go to the backend. Then to Settings -> General then change the address:
WordPress Address (URL) and Site Address (URL) from http:// to https://
One will then have to login to the site again. Clear the cache to see the new result.

– Install the WordPress plugin Better Search Replace to locate the leftovers of http.
Search for http:// and replace it with https:// . Select all the tables listed. Do a dry run first then uncheck the dry run to replace all the instances of http:// with https:// .

A few more detailed resources:

enshrined.co.uk/2016/02/24/moving-wordpress-onto-https/
designmodo.com/wordpress-https/