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There are two methods to achieve this, the first one stores the filled data in the web browser's storage, your users need to open the form in the same browser in order to get the pre-filled data, here is the guide to apply this method: http://www.jotform.com/help/227-How-to-Enable-Auto-Fill-Feature.
The second method allows to continue filling the form using a different device, and to apply this method you will need to create a first form where you will get some data from your user and will redirect to the second form, this is a session method and you need to use page breaks to save the filled fields, on this guide you will find the steps to apply this method to you form: https://www.jotform.com/help/97-How-to-Save-Forms-and-Continue-Later.
Feel free to choose the method that better fits your purposes.
Hope this helps.