To remove an assignee, simply untick the checkbox corresponding to that option. You can also click on the tiny X icon in the assignee box. Once done, click on Post.
To add an ID, tick the corresponding checkbox and select Done. To remove an ID, untick the corresponding checkbox and select Done.
You can also input your own tag(s) by selecting the Enter Custom Tag option. A pop-up box will appear asking for the new tag name, click on the Add Tag button once done.