Over the weekend, rstan 2.9.0-3 was released on CRAN and its binaries have mirrored out. 2.9.0-3 is essentially identical to 2.9.0-2, except there were some changes made in a failed attempt to get more information about crashes that occur with optimizing() or vb() on SPARC hardware. As can be seen from
https://cran.r-project.org/web/packages/rstan/NEWSnot much changed relative to 2.9.0 but there were some fixes to bugs that were reported about a month ago. You should be able to update to it simply by executing
but see
https://github.com/stan-dev/rstan/wiki/RStan-Getting-Startedif you are installing rstan for the first time on that computer.
Ben