Welcome to the forums Bruce!
First I want to clarify what "updates" mean in Linux Mint. You are using Mint 18 which is based on Ubuntu 16.04 Xenial. This is a "Long Term Support" (LTS) distribution, which means the goal is to provide stable and well-tested software that will last for many years. The "16.04" indicates April 2016. So therefore, Mint 18 uses stable software from April 2016. Mint 18 will be supported through the year 2021, and the software will always stay at those April 2016 versions (with the exception of security updates and so forth).
I can check
https://packages.ubuntu.com/xenial/cherrytree and see that the correct and supported version of cherrytree for Ubuntu 16.04 and Mint 18 is cherrytree 0.36.4. So if your Mint 18 system has cherrytree 0.36.4 installed, then that is the correct version, Mint didn't miss or skip any updates. If however, you have an older version than 0.36.4, then that's a problem we can help you troubleshoot.
I hope that made sense? Good. The next part of your question is, what if I want to replace a stable, long-term support application with a newer version? The answer is that, yes, this is possible, and there are several ways to do it. A popular method is to use a "PPA" (Personal Package Archive) which is basically a type of third-party software repository, unsupported and untested by Ubuntu and Mint developers). You'll have to decide for yourself the pro's and con's of using older, stable software vs. newer software. Sometimes it is a no-brainer, for example if you need the latest version of an application to meet a work deadline.
So you've decided that you are comfortable with the risk of upgrading cherrytree to a newer version, and you've decided the cherrytree PPA is the way to do it. The most important question is, how? My recommendation is to follow the directions "Adding this PPA to your system." If the instructions aren't detailed enough for you, then you can click "Read about installing" to get more detailed instructions (with screenshots). Let me know if you run into any errors or problems, and I'll be glad to help.
Here's the link to the cherrytree PPA page, with instructions:
https://launchpad.net/~giuspen/+archive/ubuntu/ppa