You can edit a Library Category via the administration area of an Activity or Event (see find content in Content Manager or Find Learning or view an individual Event if you're unsure how to do this).
Click on the Catalogues tab on the left-hand side.
Scroll down the page to the Library Category section.
Hover over the Category and click on the pencil icon that appears.
Make the changes that you need within the popup window.
Scroll down within the window and click on the Save Category button at the bottom.
Click on the word Close next to the button.