If a mistake has been made with the data that has been entered during a running workflow, it is possible to correct it at any stage. First select the workflow instance that has the incorrect data. Then right click on it to bring up a context menu.
Then select "Alter case", this brings up a dialog which allows the case to be edited directly.

For example in the case of the holiday request workflow, used throughout this how to, the dates requested for holiday can be corrected during the request process.