The 5.6.11 was a Kitkat Rom, right? The 5.12.17 merge the two system patritions.
1. Install MIUI 5.12.17 via Updater App (or via Bootloader, if you don't have the installer)
2. Install Install MIUI 5.12.17 via Updater App again. (not via Bootloader)
3. Install smiui 6.2.25 via Updater App
4...