My Smithing is at 100 and I have improved a Daedric sword. How do I improve it a second time? It doesn't let me. Do I have to level up?
If your Daedric sword is at legendary then you can't improve it anymore without using items and/or potions to (temporarily) increase your smithing level past 100.
From Elder scrolls wikia:
"The table shows what quality improvement an item will achieve based on Smithing level and if the Smithing perk for the item is taken or not. Items can still be improved once they reach Legendary, but the description will not change. Each additional quality level beyond Legendary improves Armor by 3.6 and other items (weapons, helms, boots, etc.) by 1.8.
Alchemy and Enchanting can improve results significantly via the Fortify Smithing effect allowing for greater improvements at a given level than the table shows. Combining both alchemy and enchanting to improve one another and then combining both to improve Smithing as much as possible can be very powerful."
So in order to increase the stats on your sword, you should wear a few items that increase your smithing skill and/or drink a potion that will increase your smithing skill.
If your skill is raised enough, you should get another option to improve your sword again.
It will still keep the Legendary tag but the stats should be increased.