Update instance number

2

The instance number of our process contains a field that is settup by the user in the form at the first step.
What do we do when the instance is automaticaly started by hotmailbox activity in the fisrt step. In that moment we don't have that field filled in yet. A user will take that instance and move it forward after filling in that field. How can we update the instance number in that instance and avoid instance numbers like in the second screensot?

The documentation seems a little confusing too (See attachment)