Hi Shawn, you would normally just select it and run the Fillet command.

However, fillets can be difficult to calculate on pieces that meet at a very shallow angle - the fillet surface can end up being very small in size in that kind of a case.

How was that model created, was the top sliced off of an extrusion with a cutting surface? You may get a better result if you fillet the edge in the cutting surface first before using it to slice the top off of the boolean, rather than waiting to do the fillet in the final end result in this case.

Also can you possibly post your 3DM model file instead of only a screenshot? A model file helps a lot to be able to experiment with, and do things like zoom in and examine what you've got, which is not really possible to do with just a screenshot.

- Michael