Changing the license of a post after the fact is not currently supported. Limited support for license changes has been discussed, but doesn't currently seem to be slated for implementation.
Currently, your best option is probably to delete and re-create the post with the correct license. Note that this will still allow users with (currently) the Curate ability to see (and therefore use) the post that's under the original, unintended license.
See for example Allow licence change when there's only one contributor and auto re licensing.
You can also set the default license to "make me choose", forcing you to explicitly choose the license for each of your posts before saving them. While that of course can't prevent you from making a choice different from what you intended, it will at least ensure that you don't forget to make a choice.