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You use the Edit Mode when you are editing an existing issue or creating a new issue. To edit a value displayed in the Structure widget, do one of the following:

double-click that value;
select the issue and click Edit button on the toolbar;
select the issue and use a keyboard shortcut – Tab or F2.

If the value is a link (like in the Summary or Assignee fields), you can still double-click it: the browser will not open the link but will start editing instead.

If you are already in the Edit Mode, you can simply click the value you need to edit, or navigate there with special keyboard shortcuts (see Using Keyboard in Edit Mode).

In the Edit Mode:

a field editor is shown in the currently edited cell;
the edited column is highlighted in the table header;
Edit button on the toolbar is toggled on.