You can customise existing mini-apps by adding additional assets to them when in 'All mini-apps' mode.
Simply select the required mini-app in the Mini-apps tab and click 'Upload File' in the right-hand sidebar. Now select a file to add to the mini-app and click 'Upload file'. This will add the file to the mini-app and this new content will appear in its list of associated files. Clicking on one of these files will open the file for editing.
The files which constitute a specific mini-app are stored in a single folder. Different versions of existing mini-apps can be created by adding a file to their folders, which will then be saved as the latest version.