This article explains how to set the privacy of an event on Facebook. While it is not possible to change the privacy of an event, it is possible to duplicate it (guest list included) to make it public.
Steps
Step 1. Go to https://www.facebook.com in a browser
You need a computer for this method.
If you are prompted to log in, enter your username and password at the top right, then click "Log in"
Step 2. Click on Events
It is located in the left panel, inside the "Explore" section.
Step 3. Click on the event title
Step 4. Click on ⋯
It is located to the right of the "Edit" button, under the cover image.
Step 5. Select Duplicate Event
A new window will appear.
Step 6. Select Public Event from the drop-down menu
It is located at the top left of the event window.
Step 7. Enter more details about the event and click Create
Everyone you invited to the original event will receive an invitation to join the new public event.