Download Salesforce Change Set Package.xml  extension in your chrome. Using the below URL

https://chrome.google.com/webstore/detail/download-salesforce-chang/olkmefomaellbafiabkljcemiljkkbeh 

Now you’ll get the option to Download package.xml on inbound and outbound Change Sets 

  • Go to the Sandbox Setup

  • In the Quick Find box entry Change Sets and select Inbound Change Sets or Outbound Change Sets option 

  • Now open the change set, here you can see the Download package.xml button, click on the button

  • Now your package.xml is downloaded