Hello, 
 
Thank you for reaching out to us. 
 
Under the Set up->More->Advanced section you can enable the Allow respondents to edit delivered submissions at a later time option.
 
To make sure that your clients can edit the submission multiple times, I would recommend you to add the Edit submission variable under the email notifications sent to your clients as seen below. 
 

 
In case you need any other assistance, please let us know.