I want to add SSL Support to my wordpress site on GAE, I read this post and what I can't figure out is how to add the Handler to my app.
I want to know what's the way to install and get the cert renewed automatically every 3 months.
It is currently (2017-05-01) not possible to automatically renew the certificate on App Engine since Google is lacking the appropriate APIs on their end to update the certificate.
They are currently looking for alpha testers for an upcoming API that should address this issue.
You can follow the open issue here and also submit the form to sign up for App Engine Managed Certificates.
As a potential alternative, you could use Cloudflare's free universal SSL. Please note that you will have to use the flexible option meaning that the connection between Cloudflare and App Engine is potentially vulnerable. Depending on your use case, this may or may not be acceptable.
You can find the detailed instructions here.