Really simple to do in base paint.net with no plug ins
Copy the image, duplicate it onto a new layer, use the hue adjustment to adjust the hue until the red is your desired blue, use the eraser tool with a very low hardness (like 0%) to erase the mis-colored "skin" or "crust" at the bottom of the image so that the correct color comes through from the layer under. Flatten and export.
Don't try to select the red part, because you'll just get ugly isolation errors because paint.net doesn't know how2boolean
