I might have a (small?) blindspot, however the documentation says: Menu option 42 will do the upgrade.. Note that you have to be on the snapshot version of openhabian to get the correct option for that!

[openhabian] Moving to OH3

mstormi (Markus Storm) November 25, 2020, 1:41pm 2

Not snapshot. On the master branch for the time being.
As told you the announcement.