How to change the default font in draw.io

Currently it isn't possible for this shape. In the next release we'll add address and tab parameters. Generally you can do this for most mockup shapes: select a shape, go to format->style and in this window you can change optional parameters. The dialog is a temporary solution and there will be a better method soon.

It depends, see below. Some browsers don't provide the permissions to perform a folder selection when downloading a file, browsers are very restrictive about file system access.

For these browsers, the only option here is to change the download folder in the browser options while using draw.

If you use Chrome 83 or later draw.io supports Native File API, which will open your operating system save dialog, allowing you to save anywhere.