iOS Delta Chat v1.16.0
The “import backup” button should be on the start screen besides “connect to server” and “scan QR code”.
To import a backup you first have to select “connect to server”, which IMHO is not obvious. My first impression was that importing backups is not possible at all on iOS.
The Linux client does it as expected: “add mail account”, “scan QR code” and “import backup” are on the same screen. Same for Android client.