Generic answer. Restore from the backup you took just before installing the new software.
Generic answer. Reinstall the old software from the old software's installation media.
Generic answer. If the new software happens to be a new operating system version, make a full backup (Carbon Copy Cloner or SuperDuper), erase the disk using disk utility, install the old operating system version, use migration assistant to bring your personal stuff back from the full backup you made before starting.
If you give more information, perhaps someone can give better advice.