Hi Sven,
you can download the appliance update files when you choose the checkmk appliance in the top right of the download page :
Older versions can be found in the download archive:
You will have to go through all majors versions, so first update to 1.4 latest, 1.5 latest and then 1.6 latest.
Additionally you will have to install matching checkmk versions for the appliance version after the update.
The update is documented here :
And As always - make a backup first 