You can (and should) update individual packages on Ubuntu through the standard mechanism. However, you cannot do a full release version upgrade, e.g., from 14.04 to 16.04 or 16.04 to 18.04. This will badly break everything since the release upgrade mechanism has no concept of how boot works on a TX2. If this is the case, then I’m afraid you need to do a full flash.
If you have data or something critical on the root file system which is critical, then you can clone first to save the data.
If you want Ubuntu 18.04 LTS, then use the SDK Manger version 4.2 (sort of the replacement to JetPack).
FYI, the image is not accessible (permission denied).