Updating Post Metadata via Custom Meta Box

I’ve created a custom meta box with only one input: a text field.

It saves to the database successfully, and I’m able to display it on the front end.

But how do I update the post’s metadata using the custom meta box that I created in the admin panel? Once I’ve added a value to it, it disappears into the database, and I can’t see it in that panel anymore.

Ideally, I’d like the current values in the field to show just below the input (exactly the same as if they were simple tags), so that I can delete them by clicking an “X” next to each one.

Please help. All tutorials on the subject completely ignore the subject of how to update custom metadata in the admin panel once you’ve created it.

Thanks!