This assumes you've followed a previous post I made on deploying ghost on dokku.

The upgrade path is actually quite simple.

In the Dockerfile, update a line that looks similar to this one:

FROM library/ghost:1.6.0

And replace that version string (e.g. 1.6.0 above) to whatever is listed as the latest version on the library/ghost page. You'll want to put in the specific version string to force dokku to redeploy.

At the time of this writing, the latest is 1.8.6. So as an example, your Dockerfile will look like this:

FROM library/ghost:1.8.6

COPY config.production.json /var/lib/ghost

Commit this file, then git push dokku. There you have it; you should see that ghost has updated.