If you simply want to remove a tag from an asset, you can just click on the tag again.
if you want to remove a tag completely, you need to remove that tag from every asset and afterwards it does not appear anymore. There is currently one possibility to remove a tag completely and this has to be removed one by one.
You can do it by removing the tag from every single image the tag is on. To do so, you can search in the the backend of the library by tag:
Then select all the images that are in the result. And with the bulk action on the top right remove the tag: