Most of the older versions are not supported anymore (except 1.2 that'll still be supported for a while). So downloading one of this versions is on your own risk (
because security bugs are NOT fixed there anymore)
All download links can be found on our wiki:
http://wiki.mybboard.net/index.php/Versions
During this six-month period when we are still supporting 1.2.x, we may release security vulnerability patches, but no general bug fixes will be released. All users who insist on using the 1.2.x branch are highly recommended to upgrade to MyBB 1.2.14 if possible and apply the patches that are released in the Announcements forum here. At the time of writing, there is already 1 security patch available for 1.2.14.
The MyBB Group recommends all new users to install the latest version of MyBB from the 1.4.x series in order to have the latest features, bug fixes, and security updates.
Regarding downgrades from 1.4.x to 1.2.x: There is no official support for this. There is no tool released to do this. It is not recommended to attempt this process.