. It is better to call off the engagement if you are not too sure about having a happy married life with her before it is too late,
2. First make a police complaint about her threatening to commit suicide if you do not mayy her. This may save you to some extent in case she takes any drastic step,
3. Then talk to her and her parents informing them politely that you do not find that this alliance will work for long for which you want it to be called off,
4. Rest assured that no body can force you to marry her if you are not willing to,
5. Return all the items and money you have taken, if any, duly keeping records of your return & their receipts,
6. She may file a case of having S*X with the promise of marriage but refusing to marry lateron which will be considered as rape. Contest the case on its merit, if filed..