Hi and welcome to the forum!
If the quilting isn't too far along, I would rip it out and then add your border and quilt it. I would use the blue border to off set all the red too.
You could use a razor blade to undo the stitches, but go slow because one quick move and you'll cut what you don't want to. This is how I ripout seams.
I'm sure others here could give you more help.