Unfortunately, we do not provide older versions of the platform for download, so your only option at present is to upgrade to the latest release of the platform unless of course you already have the files to upgrade to from an earlier release. Upgrading to an earlier release however can have downfalls even in that scenario, in that if you have an issue, you would not be supported (you would be asked to use the latest release), you would be using a version with known bugs, and more importantly, you would be upgrading to a version which has known security issues that have been resolved in later releases