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.
Apologies, forgot to upgrade you to Java 8.
Download this: h//ttp://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html and look for 'Mac OS X x64 221.98 MB jdk-8u45-macosx-x64.dmg'
Install that file.
Then do the ./run.sh
I have followed the instructions and I am now getting the following errors:
(sorry couldn't put in code as the forum blocks newbies posting links)
2015-06-07 18:35:26 INFO: Nxt server 1.5.10 started successfully.
2015-06-07 18:35:26 INFO: Copyright © 2013-2015 The Nxt Core Developers.
2015-06-07 18:35:26 INFO: Distributed under GPLv2, with ABSOLUTELY NO WARRANTY.
2015-06-07 18:35:26 INFO: Client UI is at h//ttp://localhost:7876/index.html
2015-06-07 18:35:26 FINE: Peer 89.250.240.60 version null returned error: {"error":"Maximum number of inbound connections exceeded"}, request was: {"protocol":1,"application":"NRS","requestType":"getInfo","version":"1.5.10","platform":"PC","shareAddress":true}, disconnecting
2015-06-07 18:35:31 FINE: Known peers: 268
2015-06-07 18:35:33 INFO: nxt.apiServerEnforcePOST = "true"
2015-06-07 18:36:27 FINE: Peer 89.247.147.183 version null returned error: {"error":"Maximum number of inbound connections exceeded"}, request was: {"protocol":1,"application":"NRS","requestType":"getInfo","version":"1.5.10","platform":"PC","shareAddress":true}, disconnecting
2015-06-07 18:36:48 FINE: Peer 37.120.168.204 version null returned error: {"error":"Maximum number of inbound connections exceeded"}, request was: {"protocol":1,"application":"NRS","requestType":"getInfo","version":"1.5.10","platform":"PC","shareAddress":true}, disconnecting
2015-06-07 18:37:08 FINE: Peer 46.108.156.144 version null returned error: {"error":"Maximum number of inbound connections exceeded"}, request was: {"protocol":1,"application":"NRS","requestType":"getInfo","version":"1.5.10","platform":"PC","shareAddress":true}, disconnecting
I can access the login screen h//ttp://127.0.0.1:7876/index.html however on entering my passcode nothing happens.
Please advise,
Thanks.