Just click the preview inside a node. If you want to export the final result of the thing, use the last (rightmost) node (the one that has no connections coming out of it, only connections going in).
Once you've clicked the preview inside a node, a big file explorer will pop up, select the folder in which to save, and type in a filename in the textbox at the top, below the line that shows the folder path. There is no text cursor, so it's hard to know you can type in there. AND don't forget to add .png at the end of the filename!
Then click the little save icon to the right of that textbox, which will save it but won't indicate that it did. I think it should then close the file explorer if it's saved. If there is an existing file by the same name, it'll turn the filename red, and if you click the save icon again, it'll overwrite the existing file.
If you don't want to save, you can close the file explorer thing with the little X above the save icon.
It's kind of a UI nightmare.