pkg - reverting an upgraded port to previous version

Scanning the Pkg-manuals i cannot find a solution to delete an upgraded port by pkg upgrade and reinstall the previous version of a single port with pkg. Is there a method using Pkg or how should this be done?
you're not going to find a solution for that because you can't do it. Packages come in sets. You'd have to find (or create) a repository that has the version want. These packages are all inter-related by exacting dependency specifications.

Your best bet is to build your own repository using poudriere or synth with a manually downgraded port in the ports tree you use.