I have had to do this a few times so will try to document the process I followed. If you have numerous datastores then make a note of which datastore the machine sits in. The next step is to unregister the VM from vCenter / ESXi
Then you need to enable SSH on your ESXi host
Use Putty or something similar to connect to the ESXi host
Run the following commands (in italics)
cd /vmfs/volumes/<datastore name> ie if the datastore containing the VM is named datastore1 then you would run cd /vmfs/volumes/datastore1
Next run ls to check that the VM folder is there
Rename the folder using mv <oldVMFoldername> <newVMFoldername>
Change into the folder with cd <NewVMFoldername>
Rename the nvram, vmsd, vmx and <VMname-xxxx.hlog> files by running mv again ie
mv oldname.nvram newname.nvram
mv oldname.vmsd newname.vmsd
mv oldname.vmx newname.vmx
mv oldname-1234.hlog newname-1234.hlog
Rename the vmdk file with vmkfstools -E oldname.vmdk newname.vmdk
Edit the vmx file with vi newname.vmx
Change the references from oldname to newname within this file
Register the VM with the ESXi server and it should power on ok
This link gives more detail and other methods VMware KB: Renaming a virtual machine and its files in VMware ESXi and ESX