To edit a passage, choose the edit icon from its popup menu in the story map, or double-click it (on desktop computers). In the editing dialog that appears, you can change a passage's name, tags, and text. Your changes are automatically saved when you close the editor dialog. You can do that with the close button in its upper-right corner, or by pressing Escape.
A passage must not have the same name as any other one in your story; if you accidentally give a passage a duplicate name, Twine will prevent you from saving changes. Changing the name of a passage will break any links to it from other passages.
Tags do not have any specific use in Twine, but may have special meaning to the story format you are using. To add a tag to your passage, use the + Tag link beneath your passage's name. Entering a tag name and then using the checkmark icon or pressing Return will add it. (Use the X icon instead to cancel adding the tag.) As a matter of convention, Twine tags are not allowed to have spaces in them. Twine will automatically convert any spaces in your tags to dashes.
Use the text area below the tag list to enter your passage's body text. How to enter formatting such as bold or italics depends on what story format you are using, but both Harlowe and Snowman use Markdown for basic formatting. They also both allow these formats for links: