You can do it in two ways:
Add Topics -
Topics are like hashtags. They associate a context to the video. Which makes it easy to searchable and group.
Add one or more relevant topics to your video. You can select from a list of existing topics or create a new one.
Add to Library -
If the 'Library' Menu is enabled, you can add/remove your video in the existing Library by clicking on 'Add to Library'.
Each section and subsection in the Library has a topic associated with it.
When added to the Library, topics of each section and subsection where your video is added, gets added automatically.
Similarly, removing a video from the Library will automatically remove the topics which were added during addition of the video to the Library.
Also, if you have added tags to your video which match one or more section/sub-section in your Library, the video will automatically get added to the Library for those respective sections/subsections.
Note: You cannot add a new section/sub-section to the Library via 'Add to Library'.
How to add Topics to a video
Click on the 'Videos' option in the Manage menu on the home page
Click on the settings icon of the video that needs topics to be added
From here you can add topics using “Search & Add” and “Add to Library”
Search & Add: Add topics manually to make content searchable
Add to Library: To add/remove your video in the existing Library