If you open a time card that's been Submitted to you, and it isn't something you can sign off on, you can Reject it the bottom of the time card window.
First select the Submitted time card.
Scroll to the bottom of the card, and click Reject.
You'll be prompted for a note, which will be presented to the apprentice in the notification of the rejection, and then you can wait for the apprentice to resubmit.
Once the apprentice resubmits, you'll receive a notification prompting you to review it again.