You can cancel a keyword by clicking Keywords & Numbers on the left panel. To the right of the keyword that you wish to remove, click on the three horizontal dots which will display a drop down menu.
In the drop down menu, you will see the option listed as "Delete Keyword" to remove it. A prompt will then ask you to confirm if you would like to delete the keyword.
Please note that once a keyword is deleted, there is a chance that it can be rented out by another user. Once it is purchased by another user, it can no longer be added back.