Please refer below KBs for the same:
1. VMware KB: Upgrading to vCenter Server 5.5 best practices
2. VMware KB: Methods of upgrading to vCenter Server 5.5
Let us know if you have any doubt
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Thanks & Regards
Vikas, VCP70, MCTS on AD, SCJP6.0, VCF, vSphere with Tanzu specialist.
https://vThinkBeyondVM.com/about
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