Carefully turn the mask inside out. The stiffness of the interface material might make this a little difficult. Go slowly.

Be especially careful not to tear the interface material while doing this, especially at the two inner sharp angles (marked with arrows).

Flatten the mask. Run your fingers along the inside of the seam to push it all the way out, then press the seam down to flatten it.

You probably won't need to iron the flattened seam, as long as the interface material that you're using has enough stiffness.