How to clear the previous data saved on the list item before the requestor submit again

@Dolly20 when requestor updates the value, can't they remove and select the new ones?

If you still wants to go with your request then maybe you can start the workflow conditionally when approver rejects the reject and set those fields to blank using update item.

@Dolly20 ....you need to create the Nintex workflow and:

• In Workflow Settings, trigger workflow when item is modified and select Conditional from drop down and set the condition e.g. Approval Field is Rejected
• Now in the workflow add Update Item action
  • Select the fields you want to set as blank
  • and leave the value field  as empty