How do i apply service packs

I have cs2.1 and not applied any patches since then. But I have got more 10,000
users and 50,000 blogs currently active. Please help how do i apply the patches
without affecting the system. Is their any version that supports ajax technology
so that i can upgrade to it..

Before you start your upgrade, make a backup of everything (download a copy of
all files in the site andmake a copy of the database)

To upgrade your site, you'll first need to download the latest service pack (SP3
for CS2.1) and unzip the files.  I then suggest you place an app_offline.htm
file in your website informing your users that you are currently upgrading the
site and to come back soon.  You'll next need to files in the web directory of
the service pack folder into the root of your website and the files in the
Binaries directory in the ~/bin/ folder in your website.  Once the files are
uploaded you'll need to execute the upgrade script against the database and then
your site should be upgraded.  Now to allow users to start visiting your site,
you need to remove the app_offline.htm file and you should be off and running.

Before you attempt to upgrade your live site, I highly recommend that you make a
copy of the site and database on your local machine and attempt to upgrade that
so that if there's a problem (e.g. you've made some customisations that the
service pack replaces), you can isolate tehm and have a fix for them before you
go live.