building levels in russian

i am having trouble with building levels in russian

я поднимаюсь на вершина здания - i am ascending to the top of building.
how do i say 'i am ascending to the top floor'?

я поднимаюсь на верхний етаж - i am ascending to the upper floor?
did i say that right in russian?

я поднимаюсь на третий етаж - i am ascending to the third floor.
do i have the correct preposition here?

я спускаюсь в низ здания - i am descending to the bottom of building
how do i say 'i am descending to the bottom floor'?

я спускаюсь в нижний етаж - i am descending to the lower floor
did i say that right in russian?

я спускаюсь в третий етаж - i am descending to the third floor.
do i have the correct preposition here?

я поднимаюсь на вершину здания
it's этаж not етаж
'i am ascending to the top floor' - я поднимаюсь на верхний этаж .That actually means that you are going on upper floor it doesn't mean the top floor. If you want to specify that you are going on the top floor it will be - я поднимаюсь на самый верхний этаж
я спускаюсь на третий/нижний этаж (means on the surface of...)
i am descending to the bottom of building - this might be a little tricky in Russian. You could just say я спускаюсь вниз - this means both "I'm going to the bottom of the building" and "I'm going downstairs". If you want to specify that you actually going to the bottom of the building it will be - я спускаюсь к низу здания - but it will sound unnatural