You need to be a bit more specific in your problem description. What distro and version are you running, what program and version (old) are you trying to install and what are the reasons for doing that (not compatible with newer versions....)? You have the ability to install older versions of software in Linux if there is a need for it but you have to take into account the dependencies too. If you downgrade a package you might also downgrade dependencies, uninstalling the newer versions which might be needed by other software packages. Soon you'll encounter yourself in dependency hell...