At this time there is not a function to allow you to essentially un-submit your time card so you can make edits to entries on it. Once it has been submitted it will remain in a pending-review state until the reviewer accepts or rejects the timecard.
I think you actually are wanting a feature to allow someone to un-submit their timecard after they've submitted it so they can make edits. I would suggest that you submit an enhancement request to ask for this feature since it is not one that we currently have.
You can submit enhancement requests from the Home page of this client portal. Use the Submit a Product Idea service button to send it in for review.
If there is anything else I can do for you just let me know.