is it possibe to update from 3.5 to vsphere5.0 ?
Welcome to the Community - Yes you can - check out the upgrade guide for the details and resuirements - http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-501-upgrad...
Direct upgrade from ESX3.5 to ESXi5.0 is not supported. You have to do multi level upgrade to come to ESXi5.0
refer https://www.vmware.com/support/vsphere5/doc/vsp_esxi50_u1_rel_notes.html#upgrading
As said in previous posts, it's not so much an upgrade as a migration due to the move to 64bit only OS for vcenter etc and the fact that vSphere 5 is ESXi only. Depending on your infrastructure, it should be possible to do it without downtime to virtual machines however.
Just completed our upgrade from ESX 3.5 to ESXi 5.0. It was more of a migration though as all the installations were fresh. It was pretty easy though since all of our VMs are on a SAN. Just shut the VM down, attach it to the new server, upgrade vmware tools, upgrade the hardware, and your done (we also upgraded the NICs to VMXNET 3, which is optional).
