Ebooks and downloadable audiobooks will automatically expire at the end of the lending period. After the title has expired you can download it again if there is no waiting list.
In Libby, the option to renew an ebook or downloadable audiobook will appear 3 days before the title is set to expire by selecting the 'Request again' button found on your Loans page. If the title has no holds, you'll be able to borrow it again immediately once it expires. If it does have holds, you'll be able to borrow it again as soon as you reach the front of the wait list.
Log in to your Libby account online or through your app and click Loans to find a list of your currently checked out titles.