How do I have someone make changes to their form without redoing it?

Are you referring to your submitter? 

If so, you may follow use the Save and Continue Later feature, it allows your users to complete a form in a later date even in other PC, you will find the steps on this guide: http://www.jotform.com/help/97-How-to-Save-Forms-and-Continue-Later. 

You may also follow this guide and allow your users to save the data, the data will be cleared when the form is submitted and it only will work if you users complete the form in the same browser, please take a look for further details on this guide: http://www.jotform.com/help/227-How-to-Enable-Auto-Fill-Feature. 

If it does not help, please provide us more details about your question, we will be glad to assist you.