Just let us know that you want to upgrade (however you prefer, on the Piwik PRO platform, on our website, by email…) We'll guide you through the process. The length of the upgrade depends on several factors, most of them contractual.
As soon as the paperwork is down, however, the technical side of the upgrade is fast. You'll have access to the Enterprise plan right away. If you require any custom setup or implementation then the process could take longer, but access to the Enterprise features themselves is almost instantaneous.