Assuming that you mean 'Install a newer version of package X in my old version of R/Bioc', you can do this by manually downloading the correct file from the 3.0 page, then using
install.packages(<package name goes here>, repos = NULL)
But if you do this, and things don't work correctly, then it is up to you to figure out why and how to fix it. The recommended approach is to install the current versions of R and Bioconductor, which are guaranteed to work together (or if not, you will get help and usually a quick bug fix).
So long story short, don't do that. Just update.