We console Debugger View
When viewing an error in the Backtrace debugger, you can click on the download icon on the upper right. This will give you the available formats to download a crash report. 
From this example, if we want the Minidump/dmp file, select the Raw.
Morgue CLI Tool
The command is: morgue get <projectname> objectid -o /path/to/output.dmp
You can get the objectid by looking at the error within the Backtrace UI, it appears at the end of the URL when viewing an error in the debugger (debugger/objectid )
If you are unfamiliar with the Morgue CLI tool you can find learn more here: https://github.com/backtrace-labs/backtrace-morgue