The current version of Currency Server is indicated in the Change Log.
As a policy, minor updates (e.g. from v. 5.0 to v. 5.1, v. 5.2, etc.) are free, while major upgrades (e.g. from v. 3.0 or 4.0 to v. 5.0) are eligible for special upgrade conditions. Clients with a Premium Support plan that has been active since the purchase of the software are eligible for an additional discount on upgrades.
To update the software (within the same major version) you only need to download and install the latest version. Your existing license keys (if this is a minor update) and configuration will be preserved. Any 4.x add-on licenses will also be preserved and remain functional in version 5.x (i.e. you only need to upgrade the software, not the add-on options). If this is a major upgrade, a new license key can be entered during setup, or later in the Software tab. The Currency Server services may briefly be stopped as part of an upgrade, resulting in temporary unavailability to respond to client requests. A reboot is usually not required.
Major upgrades are available here. Your existing 4.x add-on licenses will continue to work even on the version 5.x Standard Edition.
If you purchased version 2.x of the software under a "free upgrades" license, or if you purchased Currency Server 4.x on or after May 1st, 2008 you are eligible for a free upgrade to version 5.0.