You can eiother replace the old map by renaming it the same as the old one and putting in hte c:\mirserver\mir200\map folder and also in you client map folder or
You can make an entirely new map by first of all placing the new map in the folder:
C:\mirserver\mir200\Map
And then going to your mapinfo.txt in the location
C:\mirserver\mir200\envir\mapinfo.txt
Enter the entry as [mapname description 0]
For example say you want to make map j15 and it is OmaCave
[J15 OmaCave 0]
Then add featyres such as light or dark by adding DAY at the end etc
Then make the mob spawn entries in the monge.txt (in the same folder as mapinfo)
And finally put the new map in your client map folder
Hope this helpes
Exercist