Why am I unable to build Up stairways in Dwarf Fort?

Solution 1:

If it are down stairs carved out using designate (This is your problem)

1. You need to channel out the stairs. Beware that if the tile above has a up/down stairs your dwarf might fall 2 Z-levels and break his leg, or neck. Try to get the mining dwarf to the current floor first, mind you need a pickaxe for channeling.

2. Now you need to construct a Up/down or up stair on that level manually. "b"-"C" and pick your stairs from the menu and place it there. Any mason will come and bring a stone or brick with him to create a stairs.

If it are constructed stairs (Just adding this since it is relevant)

1. You can remove constructions in the designate menu "d". Not sure about the specific key but you can look it up in the designate list. I think it is "x".

2. Now with the down stairs gone you can use the same method as above (2.) to construct a new stairs using construct "b"-"C".