Hello,
You have to follow the following steps:
1. Get the valid security certificate
2. Install it and make sure it is configured properly
3. Do the HTTP migration on a test server to test first
4 Set up HTTP to HTTPS 301 redirects and test it properly
5. Update all your ad code to support HTTPS
6. Ensure your analytics will work on the new HTTPS URLs
7. Submit new HTTPS XML sitemaps
For more details check- http to https migration guide