I know this is probably not the answer you want to hear, but in my opinion the best way would be to sit down with them and tell them about it (in person would be best, I would avoid over the phone). Whether there will be a massive argument or not, that depends on the family, and at the end of the day you know your parents best. But I would personally suggest you do talk with them, tell them how you feel, why you want to drop out of college - just be honest. I would not recommend other options such as dropping out and not telling them, or trying to hint the message via smoke signals or interpretive dance (as these may not be as effective).
Hope this helps, have a great day!
For what reasons do you want to drop out?
If you string together some valid criticisms of the course and college and offer better alternatives for yourself then it will help your parents see this as a mature decision. For example, saying why you dislike the college and how that is impeding your education and mental health, and why because you are on the wrong course, this is going to lead to you going nowhere. Try and have an alternative in mind, or at least some ideas of a college or education placement you would like to take part in instead. Showing how this will lead to potential career or uni paths will show them that you care about your education as much as they do. And the most important thing, try not to get emotional during your discussion with them. Remain calm, and logical, even if it gets heated. Maturity goes a long way. Hope this helps x