trying to update heroku installation of CMS ghost v4 to v5

I could use a hint: I have ghost forked here: Git for ghost v4

and deployed to heroku here: burzltag with ghost v4

• please reload page when maintenance page is displayed!

I am totally unaware:

1. how to upgrade CMS ghost from v4 to v5 on heroku installation?
2. how to install a fresh CMS ghost v5 on heroku?

Any ideas?

1a) update ghost-v4 fork:

$ heroku login

leder@home-ryzen:~/Git$ heroku apps
 ›   Warning: heroku update available from 7.60.1 to 8.0.2.
=== gerrit.leder@gmail.com Apps
burzltag (eu)
fierce-forest-49055
infinite-taiga-25466

leder@home-ryzen:~/Git/ghost-v4-on-heroku-burzltag$ heroku git:remote -a burzltag
 ›   Warning: heroku update available from 7.60.1 to 8.0.2.
set git remote heroku to https://git.heroku.com/burzltag.git

leder@home-ryzen:~/Git/ghost-v4-on-heroku-burzltag$ git remote -v
heroku  https://git.heroku.com/burzltag.git (fetch)
heroku  https://git.heroku.com/burzltag.git (push)
origin  git@github.com:leder11011/ghost-v4-on-heroku-burzltag.git (fetch)
origin  git@github.com:leder11011/ghost-v4-on-heroku-burzltag.git (push)

sync fork ghost-v4-on-heroku-burzltag

$ git pull

leder@home-ryzen:~/Git/ghost-v4-on-heroku-burzltag$ git push heroku HEAD:master