Edit Active process instance

Hello community,

I have a scenario where user will submit a request and later he have to update the value before the request is approved.

In my process model I have request form as a start form , a write node to write to request table, then a user input task which will assign task to approver. 

Now even after the task is assigned the user can go and change the number of items. This must be possible only before approval.

User should have a form where he can  edit the form he submitted. On click of "update" button the values should be updated in the process instance pro and the task already assigned should have a updated value.

Please help me to solve this.

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