We probably should look at rebuilding our appliance some time soon to include this newer version. But in the meantime, yes you can upgrade the version of Tracks in your current appliance.

As you hint, the Tracks upgrade docs aren't particularly useful to the uninitiated... Although technically, they do provide the info you need (the 'rake db:migrate' line), they're not very explicit. IMO they probably should explicitly note that you essentially need to do the first steps of an install, but then connect to the existing database and finally run the above command.

So I don't have a Tracks appliance in front of me, but it should go something like this:

If you have data in the appliance already, then be sure to do a backup. You can either use TKLBAM or some other method. You could use a "snapshot" if using LVM, or a VM product or a manual copy of the DB and files (although we'll keep the old files anyway).

Assuming that you have created your backup, my understanding is that all your data is in the database. So essentially an update is downloading the latest version, connecting to the existing DB and running the rake command. E.g. something like this should do the trick:

# download new version and unpack
cd /var/www
mv tracks tracks.old
wget https://github.com/TracksApp/tracks/archive/v2.5.1.zip
unzip v2.5.1.zip
# assuming that a new dir "tracks-2.5.1" now exists; adjust as needed
mv tracks-2.5.1 tracks
# copy old DB connection across
cp tracks.old/config/database.yml tracks/config/database.yml
# migrate DB as per upgrade docs
cd tracks
rake db:migrate
# restart services, possibly not required...
systemctl restart apache2 mysql

Now you should be up and running with the new version of Tracks...

Regardless of how you go, please post back. If you hit any issues, I can have a go on a local instance myself and see if I can work it out.