If you want to set it for all tables, go to the table style and set the Vertical Alignment (under Cells) to top. This will set it for the whole table. If there's no table style in your stylesheet, add the style by just filling out the Tag field with 'table'; don't change Class or Pseudo Class.
I couldn't see a way to set it for an individual table using inline formatting. But it's probably better to set up a special style that you can then change in one place later. In this case create a new table style but also fill in the Class field with a specific style name, so the style only applies to certain tables you apply it to.
(I apologise if you already know how to create styles. Just New UI is so new I imagine some people are struggling with the CSS terms.)