The full guide is available in vSphere Upgrade Guide: vSphere 5.5 Documentation Center - vSphere Upgrade
One of the upgrade scenario and steps that you can follow are described in the vSphere Documentation here: vSphere 5.5 Documentation Center - Moving Virtual Machines Using vMotion During an Upgrade
Basically you will need to follow the prerequisites (perform some planning, checking, updating vCenter Server).
Then follow the steps described in the scenario:
1. Use vMotion to move the virtual machines from the ESX 4.0/ESXi 4.0 or higher host.
2. Upgrade the host to ESXi 5.5, or perform a fresh installation of ESXi 5.5.
3. Add the ESXi 5.5 host to vCenter Server.
4. Use vMotion to move the virtual machines that you removed from the ESX 4.0/ESXi 4.0 or higher host before the upgrade. For vMotion to work, the hosts must be managed by the same vCenter Server instance.
If you need more details on upgrading vCenter 5.5, you can read this KB article:
VMware KB: Methods of upgrading to vCenter Server 5.5
VMware KB: Upgrading to vCenter Server 5.5 best practices
Also read this if you have time:
Installing or upgrading to vSphere 5.5 made a little easier | VMware vSphere Blog - VMware Blogs