If you have decided to break off the proposed marriage after your engagement, you should communicate your decision to the girl or her side people by posting a letter by registered post.
In this letter you hold mention the reasons clearly that what prompted you to take this decision and why this marriage cannot be proceeded with as proposed and what all items that have been exchanged during the engagement ceremony and the mode of returning each other's items/gift, sharing of the expenses for engagement, if any.
If the bride side people, on provocation, try to initiate any legal process, you can challenge them on the basis of this registered letter or communication sent to them at a later stage as and when the situation arises.