To check whether the Alexa App is already installed on your system or not, you can write the Alexa in your search box. If the app appears in the search results then the app is already installed but if it is not displaying in the results then you can download Alexa App by choosing any of the operating systems.The main difference between both the devices are the Echo has a built in speaker which is designed to listen to music, while the  Echo dot only has a small simple speaker really designed for