Or, if it's in a different directory than terrain.exe
terrain [drive letter] : [directory] [level name]
For example - terrain c:\war2\puds\multi\whatever.pud
Choose the new type of terrain for the level, (i.e. swampland) then run Warcraft 2 and load your level. Run terrain again to switch the level back, and choose the original terrain type.
Make sure to change your level back to forest, winter, or wasteland before you open it with the map editor, or it'll screw up.
If it screws up your level, don't blame me, blame the guy who made it. Who I would like to thank, for doing something Blizzard was too lazy to do. Thank you for listening to me gripe.