How do I update config.php in owncloud instance?

I set up my domain and owncloud droplet and followed this tutorial https://www.digitalocean.com/community/tutorials/how-to-install-owncloud-and-configure-owncloud-apps-on-an-ubuntu-12-04-vps to the MySQL step - but am locked out because I don’t have a login for this and can’t find where to do that step. I’m not sure if that’s necessary, but wonder if it’s related to the error message I get when going to my URL/IP address.

Title of the page is ownCloud and the ownCloud logo is shown there, plus"

“You are accessing the server from an untrusted domain. Please contact your administrator. If you are an administrator of this instance, configure the “trusted_domain” setting in config/config.php. An example configuration is provided in config/config.sample.php. Depending on your configuration, as an administrator you might also be able to use the button below to trust this domain.”

I click the button below to trust the domain, but that never sticks and I get taken to an http header site that says:

REQUEST_URI = /index.php/settings/admin?trustDomain=mydomainnamehere.com

How do I access the php config files to trust this domain? What steps am I completely missing? I do have login credentials for this ownCloud instance but have nowhere to enter them.

Also, if it’s relevant, I have not set up the SSL certificate (Gandi says I have a free one, but are not giving me that option, so I’m hoping I can wait until Let’s Encrypt releases theirs in two months).

Thanks!