Here at Employing an Apprentice, we fully understand that sometimes things in life things may not turn out to be what you expected. This goes for apprenticeships too. First and foremost, you are free to leave an apprenticeship at whatever stage if you feel the need to do so. Essentially it is the same as if you were in a regular job; your contract will state the period of notice you must give your employer if you want to leave.
However, it is important to know that you will not gain a qualification despite possibly completing a substantial amount of work if you leave your apprenticeship before completion. This is something you should consider when making your decision. You may carry this work with you to another job to complete the apprenticeship with a new employer.