The only difference between a Newsletter and an Update is that the latter feature provides a fresh free-download link for each recipient to download the very latest file you have uploaded for a digital product you are selling -- this is typically used to provide bugfixes or free upgrades to software products, updated eBooks, improved recordings of music tracks, etc.
If you have no products with a digital file offered for download, then no products will be shown to send an Update link for downloads, so you can just compose a plaintext Newsletter instead and choose to send that to previous buyers of any product you wish.