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1. Login to Service Now developers site https://developer.servicenow.com/app.do#!/share/user/content
2. Search for the application you want to download via the search field
3. Download the xml file which is the Update set for the application. 
4. Login to the instance where the application needs to be installed
5. Go to the Retrieved Update sets module and Click on the Related Links 'Import Update set from xml' 
6. Select the downloaded xml file and import the update set.  
7. Preview and Commit the update set. 
8. Application should show in the nav menu