You can change the visibility of your collection in the edit options. (See here if you're looking for changing the visibility on web, or what visibility means)
To change the visibility of your collection:
- Open your collection from the home tab
- Click 'Edit' at the top of the screen
- You'll see the collection's current visibility setting ('Public', 'Unlisted' or 'Private') underneath the word 'visibility'.
- Click the current visibility setting.
- Choose the new visibility setting. The popup will close.
- Click 'Save' at the top.
And that's it! Keep in mind that changing your visibility to public will send out a notification to your followers.