When you download a map it will be in it's own folder just like when you create one. So what ever happens to one map it won't touch any other map.
However....
If you downloaded a generic "world" map and installed it in the folder with another map named "world" already in it and you installed over it it WILL erase the existing world folder.
Just rename the maps that you create as they are just generic "world" names and then you can go to the server.properties file and add the name of the re-named world to it and it will use that folder instead.
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you can't
I dont think there is a way to erase data but you can create other sims and save them in the file of a sim you dont want anymore
Sugarcane has 2 data values (IDs) on the PC edition of Minecraft, a block data value, and an item data value. The data value for the block is 83, while the data value for the item is 338.
On the start screen, press the button combination UP+SELECT+B. This will prompt an erase data screen.
save one on the one you want to erase then when its saving turn off the power. Sadly this happened to me. :( ...