Hi Dani,
Unfortunately, you can't change owner for calendar records.
In general, you can change owner of records through clicking assign button or changing owner field directly.
However, None of them can work in calendar entity.
1. OOB assign button is not shown in view or form.
2. Owner field of calendar can't be edit.
It is not show in the form and it can't be changed through other ways.
(1)workflow: the calendar entity is not shown in the list.
(2)flow: owner field is not in the 'Update a row' action.

(3) web api: i tried to use crm rest builder tool, but owner field is not displayed.
