How to change the Previous shortcut (default alt+<-) to backspace key? I don't find this on system configuration.
It is not elementaryos policy to allow users to change existing native app shortcuts, generally. The only configurable shortcuts are in the System Settings keyboard plug.
This issue has been raised several times in the Files issue tracker (see https://github.com/elementary/files/issues?q=is%3Aissue+backspace+is%3Aclosed).
The reply of the project leader was "We don’t use backspace as a navigation key. Major browsers have dropped this convention. Closing as “Design Conflict”"