Doing a backup of all your data is always a good idea. But even without doing manual backups, Marvin stores local copies of your data in the browser you are using or your File system when you are using the Desktop app, plus there is a copy of your data on our sync servers when sync is enabled.
To do so, go to the account page through the menu at the top right.
At the top select "Backups".
Then click "select all" underneath the category list to select all all your tasks, projects, categories and notes data from your Master List.
You can also backup any settings you have and other data such as Smart Lists, Day Notes etc. by checking all the checkboxes at the bottom.
When everything is selected click the "Backup" button at the bottom to download all your data.
Make sure it is downloaded
You might have to give permission to your browser to download the file.