Firstly you need to speak to your boss or manager about withdrawing your resignation as once it has been submitted it’s a formal document stating your leaving date.
If your manager prepared to cancel your resignation and keep you on then they may simply destroy the resignation letter alternatively you might write something like:
Here is a example letter for you:
Further to our recent discussion I would like to withdraw my resignation. I now expect to remain in this area and look forward to receiving my revised contract.
Please contact me if you have any queries.