This is what your looking for:
https://my.vmware.com/group/vmware/details?downloadGroup=HP-ESXI-5.5.0U1-GA&productId=353
As far as upgrade options you have a few
1.) Wipe and install new
- This requires you to vmotion off any vm's and un-attach your storage, install as a fresh new instatllation
2.) In-place upgrade booting off the ISO CD/DVD and selecting upgrade
3.) If you have a vCenter enviroment with Update Manager installed you can import the HP ESXi 5.5U1 ISO into vCenter once you have it upgraded to 5.5 with Update Manager 5.5 and create an upgrade package baseline, then remediate it to each host at a time
Hope this has helped
