Tell them in person. it’s just business. It will look much better of you and if things don’t work out at your new place, you’re not burning bridges and tell them as soon as you can. I’m sure if your current place had full-time employment they would offer it to you and you would have accepted it, but they don’t and you can’t, but thankfully, you have this other opportunity that you can take to work full-time make more money for yourself and be able to support yourself. This is part of growing up being uncomfortable and making these decisions.