If you want to quit one of your skills....
Press S into your skill menu where it has your skills and weapon skills. Go down to engineering, click it and then down the bottom of the menu it should have engineering with how high your skill is and click the circle with a cross through it beside it, then it will ask if you want to abandon the skill.
You can still do it again but if you pick it up again your Engineering skill will be back to 0.
Hope that helps.