The easiest way I have found is with a small diamond file, like the easy lap files here.

The metal is very thin and goes quickly, so a lot of pressure or heavy grit stone is not needed, and if you use a large diamond plate or stone you can’t see what you are doing at the time.

Put the razor spine down on the bench and round off the corner with a diamond file. It helps to trace an ark with a small washer or coin (dime) with a sharpie, then follow that line with the file.

It can be done on a diamond plate or hone but you go slowly and look at the heel frequently and do be careful working with the edge up.