Over time, YouTube interface gets worse. The recent improvement separated all tags and added cross next to each one to delete it, making it impossible to select the tags, highlighting them with you mouse.
Now you can’t copy or paste tags, and when you want to clear the field, you have to click the cross on each and every tag, one at a time, which is frustrating. So, how can you copy & paste tags? The easiest way is to use mobile version of YouTube, by adding a leading m and dot before youtube.com. Here everything is like in the old days. If you have to use the desktop version for some reason, here are two options. The first one is for nerds. Open your video description for editing. Now use you browser view page source feature. Here scroll down to the keywords section, select them with your mouse, and copy paste to a text editor. Here you have to separate each tag with a comma, and if you have multiple worded tags, you’ll also have to remove the html-codes of quotes that separates them. Now copy all this and paste to the tags section in the video description. The tags should be proper. The other way is for keyboard-shortcut lover. Click you mouse in the tags field, so that the cursor will be there. Now holding Shift, use either left or right arrow buttons on your keyboard to select a group of tags. You can now use Ctrl-C and Ctrl-V shortcuts to copy and paste them. You can also use Ctrl-A to select all of them. Backspace deletes tags, also one by one, by nevertheless much faster than clicking each tag separately. Is all this frustrating? Complain to YouTube.