If you're using Crucible without FishEye, and you're not in the latest version and you have 'Update Notifications' (under FishEye Administration > Global Settings > Server) enabled, a link Version 2.8.1 is available will appear at the very bottom of the page.

If you click on it, it'll take you to http://www.atlassian.com/software/fisheye/download, which is the FishEye download page.

However, as you're using only Crucible, it should take you to the Crucible download page: http://www.atlassian.com/software/crucible/download