You can reset your skill points to their default values by clicking the Reset Points button and confirming the reset. Your skills will be reset and your skill points spent to acquire the skills will be returned. You'll be able to re-assign them.
Please note that you can reset points only a limited number of times. You initially have 10 resets, and every month you get additional free reset. If you spend them all your skills will be frozen and you won't be able to re-assign previously assigned points until next month. Please use this feature wisely.