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