There are two ways of extending the contract. In the first case, you need when drafting an agreement to register in the item "other provisions" the period of extension. Then at the end of the term, and in the absence of the statement of one of the parties of the termination of this agreement, the document will be automatically prolonged.
In the second case, you can make an additional agreement to the contract in duplicate, where to specify the validity of the document.
The contract should be signed both to managers and to assure the seals of the organizations. One instance is left, the second to give the other side of the contract. Attach it to this agreement.