manually changing alembic's HEAD position

Use alembic stamp:

alembic stamp <revision>

Delete (or move to another folder) the specific migration file (in migrations/versions folder). The head will automatically revert to the most recent remaining migration.

Using stamp will set the db version value to the specified revision; not alter the head revision number.

Based on your question, the original answer will resolve your issue. However it does not affect the head position as the title of the question states.