First of all, the codes do not say that the vent valve is bad, just the system has a problem. I hope you had a real diag from a real shop first. It needs to be smoke tested and manipulated with a scan tool to verify the failed part.
Anyway, the tank has to be dropped about a foot to acess it. It is on the passenger side of the tank at the top. It has a clip you depress and it slides forward and you need to unplug the connector and remove the hose. Make sure it goes back in the holder and clipped in or it will break when you put the tank back up.