If you made this with a SWEEP surface, then edit the profile sketch to square up the corner.
If the surface model doesn't have any history, I'd try DELETE FACE to remove the corner.
And if that doesn't work, DELETE the corner surface, both tangent surfaces and both of the end surfaces. Add all new surfaces and JOIN all the surfaces together.
But why are you doing this simple part with surfaces, when it would be much easier to do with a solid?