Upgrading your Cacti with the Plugin Architecture is basically the same as any normal install / upgrade of Cacti.
Upgrading Cacti
Once you have it downloaded, you will need to extract it. Once that is complete, just override your Cacti files with the files you extracted. There is no need to re-import your database, or anything else.
Your Cacti files are now updated.
Upgrading the Plugin Architecture
This step is actually already well documented already, just follow the normal steps to install the Plugin Architecture.
Plugin Architecture Install
The easiest way is ofcourse just to override your cacti install with the pre-patched files, but a patch is also provided.
The After
You must now re-edit your include/config.php to point to the correct database again. You will also need to re-add the names of the plugins you are using. Now, when you open Cacti, it should ask you to upgrade your Cacti database as normal. Your Cacti is now completely upgraded and ready to go.
If you are using the CactiEZ CD, instructions and a script have been provided by egarnel to do this for you with no hassle.
CactiEZ Upgrade Instructions
Errors
If after upgrading Cacti, you get this error, then you didn't properly update your include/config.php file to point to your database.You have created a new database, but have not yet imported the 'cacti.sql' file. At the command line, execute the following to continue:
(This document will more than likely be updated over the next few days)