shortening blade to fix frown from tang side

Hey what should I use to shorten the blade from the tang side? There is an uneven hone on the edge, about 2 inches are level and 0.5 inches is about 2-3 mm shorter.

Should I flat file, round file, or how to shorten blade?

Thanks, was hunting around and didn't see what I should do. Here is a pic of it from another post.

Ern 1720 antique store find, first hand sand.

I would tip it to a 45 degree angle on a DMT 325 and hone heavy on the heel until it starts to take on a smile, then gradually lower the blade using sweeping X's to get the bevel close enough to move up in grit for final bevel set.

Keep us informed of your progress with photos. It would be interesting.