Ask support to remove the sandbox and then run the app again in the sandbox. Your latest data should then be in the sandbox
I created a module that allows you to backup and restore a chain of objects. The appstore deeplink does not work, but searching for 'backup' will return it.
Full details are in the module documentation, easier to read on GitHub: https://github.com/synobsys/mendix-ObjectBackupRestore
Basically you create a configuration to tell the module what to backup. You can then download the backup as a zip file and upload it into your sandbox. I use it all the time to update a sandbox db.
Released for Mx5 but it does work on Mx6, maybe you need to fix a few layout issues to get to work.