I didn't understand any of that.
I am running on OS X and have version 1.4.18.
Can you please tell me plainly how to upgrade.
Thanks.
Here ya go:
Download this: h//ttps://bitbucket.org/JeanLucPicard/nxt/downloads/nxt-client-1.5.10.zip
Download this: h//ttps://www.dropbox.com/s/djmouly8wlts5gg/nxt_db_peerexplorer.zip
Double click the nxt-client-1.5.10.zip file. It will be in your Downloads folder.
Double click the nxt_db_peerexplorer.zip. It will also be in your Downloads folder.
Drag the 'nxt_db' folder into the 'nxt' folder. This will copy the NXT blockchain into the new client.
Open your terminal app. Type in
cd Downloads/nxt
./run.sh
Open Chrome/Safari/Firefox and type in 'localhost:7876'. Input your address, log in, and wait for about 10 min. You are rocking. 
Thank you.
Will follow that now.
I followed these instructions, however on running I get the following:
Exception in thread "main" java.lang.UnsupportedClassVersionError: nxt/Nxt : Unsupported major.minor version 52.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:482)
It then takes me back to the command line.