At a very high level:
1. migrate export db.tgz (from Windows mgmt)
2. Fresh install R80.20, copy db.tgz to new system
3. After first-time wizard: migrate import db.tgz
By default this will take the configuration but no logs.
The documentation Tal linked above has more details.
However, I also second the recommendation to go to R80.40 as it is the widely recommended release and has significant improvements from R80.20.
Just make sure you take the migrate export using the migration tools for the version you wish to migrate to.