Most programs and websites that require Java won't stop working as you upgrade Java from security patch to security patch, and even from major version to major version. On occasion, however, an old application won't run correctly with the latest version of Java. If possible, you should update or replace the application instead of downgrading to an older version of the Java Runtime Environment, but if you absolutely need older copies of the JRE, Oracle offers them for download.
Uninstall the Current Java Version
It's possible to install multiple versions of Java at the same time, but programs automatically use the latest version, so you should remove your current version before installing an older copy. Java uninstalls through the Programs and Features Control Panel, like most other Windows programs. Find the Control Panel by searching for it on the Start menu or Start screen, or in Windows 8.1, right-click the Start icon and select Programs and Features. Search for Java in the Control Panel's search bar. Select each installed version in turn and click Uninstall.
Download and Install the Old Version
The regular Java website doesn't provide a back catalog of old Java versions, but they are available in the Oracle Java Archive.
As of publication, Java 8 is the latest major version of Java. To downgrade to the final release of Java 7, visit the Java SE Runtime Environment 7 Downloads page to find copies of versions 7u79 and 7u80. Oracle recommends that most people who need Java 7 use 7u79 rather than 7u80.
Alternatively, to download an older release for Java 7, or an older major version, visit the Java Archive's front page and select a version, such as Java SE 6.
Scroll down on the page to find the list of downloads for the selected version. If you're browsing from the archive's front page, you'll see an expansive list of releases for each version, such as 6u45, 6u44, 6u43 and so on. Pick the highest number in order to include as many security fixes as possible.
Select the Accept License Agreement option near the version you want to download, and then click the download link for the file you need. For Windows, you'll usually need the file ending in windows-i586.exe. If you need the 64-bit version of Java -- not commonly used, even on 64-bit computers -- select the file name ending in windows-x64.exe.
After selecting the file, you'll need to log in with an Oracle account, except in the case of downloading versions 7u79 and 7u80. If you don't have an account, click Create Account to make one for free and download your file.
Run the installer from your browser's download folder to start installation. Installing an old version only requires a click of Install, just like the current version.