You'll see the Renew option for a Libby/OverDrive title three days before it expires, as long as there are no holds. If the title has holds, you can't renew it, but you can request it again.
In My Account – find the title in Checked Out Titles and look for an option to renew.
In the Libby app – find the Manage Loan button for the title in your shelf and look for the option to renew.
In the OverDrive site – go to the Loans page and find the title; look underneath the download buttons for the option to renew.