How can I change the host name site collection from http:// to https://?
I checked this https://blogs.msdn.microsoft.com/svarukala/2014/05/06/change-host-named-site-collection-url/ but it's not help.
First, Try to change the Site Collection via PowerShell as shown below
$site = Get-SPSite -Identity http://sp1 $site.Rename("https://sp1")
Second, Make sure that the URL is working first inside the server properly.
Note: Make sure you already add the hostname in your drivers folder by Navigating
C:\Windows\System32\drivers\etc, Open hosts file via appropriate editor > add the current server IP > click Tab from keyboard >add the hostname.
In case, it's worked inside the server but not worked outside. so you should check with a load balancer and network team.
In case it's not worked inside the server,
Backup-SPSite -Identity <SiteCollectionGUIDorURL> -Path <BackupFile>