Managing Machine (Device) Activation Limits
You can keep your eText on any computer or device to which you have downloaded it from six years to perpetuity, depending on the publisher. You can use the VitalSource app on up to two computers and two mobile devices at any one time. A record of these computers and devices is stored with your account information. Follow the instructions on the VitalSource Machine Activation Limits page if you want to de-authorize a computer or device so that you can download Bookshelf to a different machine.
If you plan to update your system software, you will have to deactivate your device (see above link) and sign in again.