You can export a dashboard by clicking Dashboard (or the name of the dashboard if it has been customized) in the header and then selecting the Export... option in the dropdown menu:

This opens the Export panel, which gives you the choice of PowerPoint or Excel formats. For dashboards with tabs, it also contains a "Tabs" control.
Selecting exporting options
The following describes the various options you can select.
The default, which is to export all tabs (i.e., the entire dashboard):
PowerPoint

Excel

To export just the currently viewed tab:
PowerPoint

Excel

You can also a custom selection of which tab(s) you want to export.
Additional information
For dashboards without tabs or for exports triggered from the deck view, these tab options won't appear and the export will always include the entire dashboard/deck.
You can choose which tabs to export, whether you're exporting to PowerPoint or to Excel.
If you are exporting a dashboard container that is organized into tabs, those tabs will be exported as Chapter slides with the name of the tab, making it easier to navigate the presentation.