Tank vent is usually behind the top handlebar on the clutch side,
behind the recoil cover, or up top below the filter area in the
inbox, I do not think its up top on the saw in question.
You have to remove the top handle reach in with pointy
pliers and pull out the vent after that handle bar is off,
its can be in a small black shroud to keep it safe.
You have to pull the valve out with the shroud on it,
then take the shroud off if one is present, put the shroud
over the new valve and push back into the pipe, there is a small
spring clip that needs to be pulled up the pipe to rest on the fat part
where the valve is to keep the valve in place.