Whether you are allowed to edit tags associated to a discussion, depends on the permission settings of the forum you are writing in. Here at discuss.flarum.org you can change tags of the discussions that you startet yourself, as an admin in your own forum you can change the tags of any discussion.
To do that in a tag view (or All Discussions view) click on the vertical three dots at the right side of an entry. Inside a discussion click on the little arrow next to the Reply button.
To remove an existing tag, just click on one of the selected tags at the top. To add a new one, click on one of the options in the list below.
