I am in Dallas,Tx and need to replace rotted wood fence post with metal post. Fence is 6ft privacy cedar fence. Should the concrete be poured to the top of the ground or below ground level. I understand the posts will get loose quicker if they are at or above groung level. Is this correct?

The reason for putting concrete around metal post is to increase its size. A 1 1/2" diameter post 2 foot in the ground can easily be pushed over. There just isn't enough surface space for the earth to push against. Putting the concrete around it increases its size and increases the surface space pushing against the earth many times. The wood post which is usually 4"x4", has a lot more surface to push against the earth.

Therefore, the concrete should be put in the hole around the post.