How to install Cumulative update or Hotfix on SharePoint Server:-

SharePoint Server Cumulative :_
Prep your Farm for the Upgrade
Pre-requsite Steps for installing SharePoint Cummulative Updates
Make sure SharePoint version is sp2 or sp1 (atleast)
Backup SharePoint Farm
Quiesce farm the SharePoint Farm from Central Admin --> Operations - Quiesce farm Stop the world wide web Publishing service from services.msc
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Package Installation
Run update packages on servers in the Farm in following order :-
(A) Web Application Server – Excel Services, SSRS, BDC – Services Server
(B) Index Server
(C) WFE
Repeat both packages Install on all servers in the farm in the order specified above.
WSS Package
Run the following package - A Reboot is required (be prepared as Microsoft is not correct on Reboot)
wss-kb974989-fullfile-x64-glb.exe
REBOOT
MOSS Package
Run the following package - A Reboot is required (be prepared)
office-kb974988-fullfile-x64-glb.exe
REBOOT


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Run the SharePoint Product & Services Technology - psconfig (Wizard) on all the SharePoint Servers in following order
1. WFE which host the Central Admin
2. SharePoint Application Services Server
3. SharePoint Index Server
4. Other WFE's
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Central Admin --> Operations – Stop Quiesce farm
Check the version of the SharePoint Server from Operations Tab
Central Admin - Operations - Servers in the Farm
Farm Information will list the version as - 12.0.0.6520
Also
Version on each server in the Farm - will be displayed next to Servers name