I needed to hide a column and make another column required in a document library. The option wasn’t available when I just tried to edit the column. Thanks to a forum post on a paid service, I figured out the process.
1. Go the document library’s setting page.
2. Click “Advanced Settings”.
3. Check Yes for question “Allow management of content types?”. Click OK.
4. Now you should see a Content Types section on the settings page. Click the only one (most likely named “Document”) there.
5. Click the field for Description and you will see the “Change Content Type Column” page.
6. Change Optional to Hidden (or required if you want that instead). Click OK.
7. (Optional step) Go back to picture library advanced settings page, Check No for question “Allow management of content types?”.