When in the APEX builder, keep an eye out for this region on the right hand side - if you haven't heard about a patch through the copious amounts of blogs or social media, then this is on place you might see the prompt.
download page, you can either download the entire product; for English only; or if you're already on 4.2.0, you can download the 66mb patch set from My Oracle Support - patch #14732511
The patch set notes can be found here, or as part of your download.
My first step to upgrade the instance on my Windows 7 laptop was to terminate APEX Listener with Ctrl-C
Now open a command prompt, change current folder to where the patch is, connected to SQL*Plus and ran the install script
conn sys as sysdba
Ten minutes later, I found no errors in the apxpatch.log although the log didn't capture the final output I saw in the command display.
It finished in the middle of
Application Express 4.2.1 patch
... lots of stuff
... Sample Access Control: Application ID 7600
... some more stuff
......region template 5047053830717713422
-- no more logs!
I opened f7600.sql, found the exact location but saw no reason why the log would stop.
Moving on, it came to updating the images directory when using Oracle APEX Listener, and for me it got a little obscure.
Ever since I can remember, this step involved copying the images folder to where the relevant web server sourced the documents, but the current documentation suggests creating a web archive.
If you are using the Oracle Application Express Listener, you need to create a Web Archive for Oracle Application Express imagesI decided not to do this in my circumstance - stand alone APEX listener, with no WLS/Glassfish because the documentation for this option didn't require it.
Now time to restart the APEX Listener using my batch file on the desktop, which runs
java -jar apex.war standalone --apex-images \apex4.2\images
I also checked my older post regarding APEX 4.1 upgrade to see if there was something I missed.
Opening the localhost:8080/i/apex_version.txt showed the patched version, so no harm done?
Take care when importing applications from 4.2 to 4.2.1 - you will need to re-apply the theme (see patch notes)
It will also pay to read notes regarding jQuery Mobile 1.2.0, which is part of this patch - there are some interesting widgets to play with.
Now I very much look forward to the new year, upgrading my current client from 4.1 so we can start a project building an application for a tablet.