When the approver requests changes, you're notified and the task comes back to you, ready to be submitted again.
Steps
Open the task. The activity shows that changes were requested, and the approver's comment tells you what to change.
Update the field, or add the missing document: select Add document, give it a name, choose the file and save it.
Reply in the activity so the approver knows what you changed, and select Comment.
Select Submit for approval again, choose the approver and select Submit. The task goes back to the approver, and they're notified.