You can easily add tags to features by typing any word or phrase into the default Tags field. If you need to edit, replace, or delete a tag, go to Settings -> Account -> Configure tags. You can find your tag in the list, or search for the tag by name using the Search Tags field.
Edit: Allows you to change the name and default color.
Replace: Allows you to replace one tag with another, essentially changing all the tags on any features and deleting the original tag name.
Delete: This option only appears if no features are using the tag.
You also have an option to create tags from this screen, by selecting the plus sign next to Configure Tags. From this screen, you can create a single tag by entering the information and selecting Create Tag, or create a series of tags in a recurring fashion until all are added with Create Tag + Add Another.