How do I change multiplayer map on my Minecraft server?

Yesterday, I spent a few hours making my own Minecraft sever for me and my friends to play on. I successfully got the server up and running and we wanted to play Survival Island. I downloaded the map and put the files from the folder into the "World" folder in my server folder, but it doesn't work. I can't change the map. Please help me with this, I spent all day getting the server up and running, and I can't change the map.