For a full reinstall, you need your full backup (including your app.yml file) and a fresh OS on your instance.
The instructions are here: INSTALL-cloud
It might be best to try the install on a new instance. You’ll need to add the new IP address to a temporary subdomain in your DNS. Or, if you are happy with the downtime, just update your DNS to the new IP address.
You do need to have some idea what you’re doing, otherwise you need help. Sorry, I can’t be that help.