How to remove a category from a record

This is probably very simple. I have created categories and assigned a number of them to my records. Now I want to remove one, but I can't see any way to do this. How do I remove a category tag? Thanks.

Perch: 3.0.10, PHP: 5.6.31, MySQL: 5.6.37, with PDO
Server OS: Linux, litespeed
Installed apps: content (3.0.10), assets (3.0.10), categories (3.0.10), perch_blog (5.6.1)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', ); ?>
PERCH_LOGINPATH: /perch
PERCH_PATH: /home/stringboard/public_html/perch
PERCH_CORE: /home/stringboard/public_html/perch/core
PERCH_RESFILEPATH: /home/stringboard/public_html/perch/resources
Image manipulation: GD
PHP limits: Max upload 32M, Max POST 64M, Memory: 128M, Total max file upload: 32M
F1: 3b606135b33e6a102526838f4152a807
Resource folder writeable: Yes
HTTP_HOST: stringboard.com.au
DOCUMENT_ROOT: /home/stringboard/public_html
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php