You cannot upgrade to CAM 8.1.3 as path names and certificate name changes due to re-branding of the product. However you can upgrade to 8.1.2 version. There is no functional difference between 8.1.3 and 8.1.2 at this time.
The download is available on our support site here:
Before upgrading please ensure you have a current backup of Cloud Access Manager:
The upgrade instructions are in the release notes.