Short answer:
For 10.9.0-S80, essentially a STABLE build of cPanel, it is not recommended at this time to consider upgrading Apache. That does not mean it cannot be done, but if you are inexperienced with performing Apache upgrades and/or uneasy about this process, you are likely better off waiting until cPanel supports upgrading to Apache in the STABLE build before upgrading.
Long Answer:
In the EDGE build, it is (relatively speaking) very easy to upgrade to Apache 2.2. You just go into the "Software" section of WHM and click the "Apache Update" link and follow the on-screen directions and read the on-screen warnings. It's practically a one-click install.
However, I would note that EDGE builds, while supporting many new features, are not necessarily recommended for production environments as they are not nearly as thoroughly tested as CURRENT, RELEASE and STABLE.
Just some information about upgrading Apache. If you go to any 2.x version, you no longer can have FrontPage Extensions support. Over the long-term this isn't much of an issue as Microsoft has announced that FrontPage is a deprecated product to be replaced. However, if your customers want this feature they may complain about the fact it is not supported.
Importantly, if you are intending to run Apache 2.2, SuExec and PhpSuExec will not run. This is important for those of you who are security-minded and rely on these utilities.