Whenever there is a moment that feels right to let them know, take advantage of that opportunity. Confess to them gently, but bluntly so they will understand you are no longer interested in partaking in that activity furthermore. Although they might have spent money on you, it was something new you were willing to try and you came with the conclusion it wasn't something you wanted to do. Even in terms if your parents aren't thrilled, the weight/guilt on your chest will be lifted and it will no longer be a burden inside of you. If they're worried about finances, you as a family could come up with several solutions on how to earn the money back. If you have a guitar, they can start by putting that up for sale.
In summary, I hope everything goes well and don't be afraid to be honest with them. You can do whatever you like, and if guitar isn't your thing then that is completely okay! You at least tried it out and was able to form a valid opinion on that hobby.