As far as I know, you cant change the date of a scheduled workflow, but you can simply cancel it and schedule a new one.
To do that you’ll need to store the workflow ID of the scheduled workflow somewhere (i.e. on a field of a related datatype, or perhaps in a new datatype specifically for the purpose).
Then, when your user wants to change the scheduled date you can run a workflow to cancel the original workflow (using the workflow ID), schedule a new one, and again save the new workflow ID for future adjustments.