Menu: Edit / Trim/Extend / Cut Corner
Cuts a portion off of the corner of a solid and replaces the corner with a flat face at a specified depth.
Point 1: Corner to be cut
In the Radius field, enter the depth - the distance from the corner along all of the edges that form the corner - to the flat surface.
Set a point on the corner. The corner is cut off to the specified depth, or radius.