Just hold down 'u' while clicking on the skill and it will attempt to untrain 1 point in that skill. It might take a few tries since the % chance to unlearn is somewhere around 9%. If you use trophies to unlearn the skill, it will take it one rank down and all the way down to 0%.

Also, re-assigning isn't impossible. You just have to move 1 skill rank at a time. I've switched skills on most of my glads many times just to try different things.