I have an IMac version 10.12.6 21.5 inch Late 2013 and I need to update. Help

Your 2013 iMac will support up to macOS 10.5 "Catalina."

macOS Upgrade Tips:

1. If you have older versions of job-critical software like MS Office (Office 2011or older) or Adobe Creative Suite/Cloud, they will NOT work with Catalina unless replaced with newer compatible versions. See: 32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support.
2. Some older apps have no replacements. If that is the case, or you cannot afford to buy new versions, do not upgrade higher than Mojave 10.14 until you can find out what apps can be upgraded or what new apps will do the same job.
3. This third-party app helps find your 32-bit apps and can tell you if newer compatible versions are available: Go64
4. Use Safari for any downloads. There are many reports here that FireFox does not properly handle macOS upgrade files.
5. If you see an error message that the installer is "damaged." it only means the current installer "sees" an older and outdated version of itself already on the computer. It will be in your Applications folder. Removing the older version allows upgrding to continue.

See: How to get old versions of macOS - Apple Support