I have managed to upgrade to v11. My solution is here https://discuss.frappe.io/t/unable-to-install-erpnext-v8-due-to-pip-v19/47892/6
I now understand that
$ bench --site my.domain.com backup
make latest sql database downloadable from front end, I thought I have to restore from tar file generated by $ bench --site my.domain.backup
How I upgrade it is by:
- cloning disk
- reinstall bench using easy install script
- do what I do on link above
- restore database and files to new site on v8.x.x
Now I have latest version with my old database, still have to backup and restore to newer OS: ubuntu 18.04 but I believe it is way much easier than upgrading.