Some things are not obvious in Edge Reflow. I’ve always found that modifying breakpoints that you have created is one of those things. By default, Reflow using max media queries. If you want to change your project to use min media queries, do the following:
1. At the very top right of the application long press on the down arrow (to the right of the plus symbol).
This will bring up the orientation dialog.
Here, you can change your project to use min media queries, update breakpoint labels or change the value of each breakpoint.
Simple… but, honestly, hard to find.