Logbook entries are locked when an instructor signs the entry.
Image 1. Locked logbook entry.
To unlock a logbook entry you need to delete the instructor's signature. To delete a signature, swipe from right to left on the signature. A delete button will be revealed. Select DELETE. Once removed, you can edit the entry and then request another signature from your instructor.
Image 2. Swipe from right to left on the signature and select DELETE.