Using filters, you'll be able to filter through the data to show the segments you require. 

Throughout analytics, you'll find the filter option in the secondary menu bar:

Click on Filter and you'll be presented with a pop up where you can begin to configure the filter. 

In this example, I want to filter all impression data for two devices. So I click on filter, ensure the filter type is 'Device' and then choose my device - which is 'Chrome OS - analytics' and then hit 'Add Filter'

Now the main UI will refresh to filter only data associated to the Chrome OS - analytics device. You can see the filters bar now shows the device too:

To add another filter, follow the same process but add the new device - in this example, the 'Office NUC - LED PC' and add the filter. Now the main UI refreshes to show both devices in the filters area and the data changes to only show based on the two devices:

To remove a filter, all you need to do is hover over the filter and hit the 'bin' icon to delete.

Do this for all filters if you want to return back to the overview of all data.